Update GitHub links in tests/README.md (matomo-org#14631)

Note:
Two of the links link to a 404. However I didn't remove them, because I didn't know what way you like to handle such a case.
1. https://github.com/matomo-org/matomo/blob/master/tests/PHPUnit/config.ini.travis.php#L23-27 (Line 156)
2. https://github.com/matomo-org/matomo/blob/master/tests/README.xhprof.md (Line 179)
I still think that adjusting these links is right however because /piwik/ will be a 404 sometime anyway.

tests/README.md

@@ -104,15 +104,15 @@ Otherwise, if you didn't expect to modify the API outputs, it might be that your
 
 System tests use Fixtures to generate controlled web usage data (visits, goals, pageviews, events, site searches, content tracking, custom variables, etc.).
 
-Fixtures are stored in [tests/PHPUnit/Fixtures](https://github.com/matomo-org/piwik/tree/master/tests/PHPUnit/Fixtures)
+Fixtures are stored in [tests/PHPUnit/Fixtures](https://github.com/matomo-org/matomo/tree/master/tests/PHPUnit/Fixtures)
 
 #### OmniFixture
 
 We also have an OmniFixture that includes all other Fixtures. OmniFixture is used for screenshot tests to provide data across most reports. 
 
 #### Keep OmniFixture up to date
 
-Remember to update the [Omnifixture SQL dump](https://github.com/matomo-org/piwik/blob/master/tests/resources/OmniFixture-dump.sql) whenever you make any change to any fixture. You can use:
+Remember to update the [Omnifixture SQL dump](https://github.com/matomo-org/matomo/blob/master/tests/resources/OmniFixture-dump.sql) whenever you make any change to any fixture. You can use:
 
     ./console tests:setup-fixture OmniFixture --sqldump=OmniFixture-dump.sql
 
@@ -129,19 +129,19 @@ The characteristics of the integration server are described in `SystemTestCase::
 
 ### Running tests on Ubuntu
 
-If you use Ubuntu or another Linux distribution, you must make one change to the filesystem configuration to make tests run fast. [Read more here](https://github.com/matomo-org/piwik/blob/master/tests/README.troubleshooting.md#important-note-for-linux-users-fix-for-slow-tests).
+If you use Ubuntu or another Linux distribution, you must make one change to the filesystem configuration to make tests run fast. [Read more here](https://github.com/matomo-org/matomo/blob/master/tests/README.troubleshooting.md#important-note-for-linux-users-fix-for-slow-tests).
 
 ## JavaScript Tests
 
 piwik.js is unit tested and you can run the Javascript tests at: /piwik/tests/javascript/
 
 ## Testing Data
 
-See [tests/README.testing-data.md](https://github.com/matomo-org/piwik/blob/master/tests/README.testing-data.md) to import testing data in Piwik.
+See [tests/README.testing-data.md](https://github.com/matomo-org/matomo/blob/master/tests/README.testing-data.md) to import testing data in Piwik.
 
 ## UI Screenshots Tests
 
-See [tests/README.screenshots.md](https://github.com/matomo-org/piwik/blob/master/tests/README.screenshots.md)
+See [tests/README.screenshots.md](https://github.com/matomo-org/matomo/blob/master/tests/README.screenshots.md)
 
 ## Build artifacts
 
@@ -153,15 +153,15 @@ You can retrieve the files generated during the build (the build artifacts) at [
 
 By default tests running on Travis CI will log all messages of at least `INFO` level in `$PIWIK_ROOT_DIR/tmp/logs/piwik.log`. In a given travis build output, you can view the logs by clicking on the line `$ cat $PIWIK_ROOT_DIR/tmp/logs/piwik.log` at the end of the build output text.
 
-Note: `DEBUG` and `VERBOSE` messages are not logged by default (to keep Travis page loading fast). At any time you can temporarirly enable logging by [modifying this file](https://github.com/matomo-org/piwik/blob/master/tests/PHPUnit/config.ini.travis.php#L23-27) and changing `log_level = info` to `log_level = debug` or `log_level = verbose`.
+Note: `DEBUG` and `VERBOSE` messages are not logged by default (to keep Travis page loading fast). At any time you can temporarirly enable logging by [modifying this file](https://github.com/matomo-org/matomo/blob/master/tests/PHPUnit/config.ini.travis.php#L23-27) and changing `log_level = info` to `log_level = debug` or `log_level = verbose`.
 
 ### Screenshot tests build artifacts
 
 The screenshot tests generated by the continuous integration server are uploaded in [builds-artifacts.piwik.org/ui-tests.master/](https://builds-artifacts.piwik.org/ui-tests.master/?C=M;O=D)
 
 ## Troubleshooting
 
-See [tests/README.troubleshooting.md](https://github.com/matomo-org/piwik/blob/master/tests/README.troubleshooting.md) for troubleshooting the tests.
+See [tests/README.troubleshooting.md](https://github.com/matomo-org/matomo/blob/master/tests/README.troubleshooting.md) for troubleshooting the tests.
 
 ## Advanced users
 
@@ -172,11 +172,11 @@ As a software developer writing tests it can be useful to be able to set breakpo
 
 ### Benchmarks
 
-See [tests/PHPUnit/Benchmarks/README.md](https://github.com/matomo-org/piwik/blob/master/tests/PHPUnit/Benchmarks/README.md) to learn about running Benchmark tests.
+See [tests/PHPUnit/Benchmarks/README.md](https://github.com/matomo-org/matomo/blob/master/tests/PHPUnit/Benchmarks/README.md) to learn about running Benchmark tests.
 
 ### Profiling
 
-See [tests/README.xhprof.md](https://github.com/matomo-org/piwik/blob/master/tests/README.xhprof.md) for help on how to profile Piwik with XHProf.
+See [tests/README.xhprof.md](https://github.com/matomo-org/matomo/blob/master/tests/README.xhprof.md) for help on how to profile Piwik with XHProf.