[mono] Integrate xharness tool (iOS)

[EgorBo]

xharness-tool works but not quite stable: it doesn't quit when tests are finished, also I don't see any file with test results.

Steps to run a test:

./build.sh -c Release -os iOS -subset Mono+Libs

cd src/libraries/System.Numerics.Vectors/tests

dotnet build -c Release /p:TargetOS=iOS /t:Test

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[premun]

Trying to fix the hang issue here: dotnet/xharness#73

[premun]

The hang is prevented here to unblock you but the app will still fail to install on the Simulator:
dotnet/xharness#74

I am tracking the installation issue here:
dotnet/xharness#69

[mandel-macaque]

Created two issues to track this:

• Notify users when we can get the xml results over the network xharness#78
• Xml results are not written in the oputdir. xharness#77

Should be easy to solve.

[premun]

So fix was already checked couple of hours ago but we have problems with arcade and cannot seem to publish the new package:
https://dev.azure.com/dnceng/internal/_build/results?buildId=613910&view=results

There is a fix in arcade apparently but it will take time. So let me know if this becomes blocker for you and I can send you a manually built package or I can push one to the feed or something that will unblock you

[premun]

@EgorBo looks like the XHarness tool will expect the app to shut down itself after the tests are done, otherwise it doesn't know if there are more logs to come. Is it something you can implement?

[EgorBo]

@premun it's already implemented in this PR, see changes in runtime.m https://github.com/dotnet/runtime/pull/35316/files#diff-8c2a0ec87d7100c3ad4942fb4d36a1d4L276

[premun]

@EgorBo if you wish, keep it as a feature and we can add the autoexit option too. But up to you, this seems as less code :)

Also, could I ask you to build a new app for me with the exiting from the dummy test file I sent you on Teams? I will do another round of testing on Monday

[EgorBo]

"In progress" legs are actually green in AzDO.