tests: On Linux, let npm test default to Chromium instead of Chrome

By default `npm test` will run concurrenty in Firefox and one of
Chromium/Chrome. Chrome for macOS and Win, and Chromium on Linux
unless CHROME_BIN is configured by the user's env variables, in
which case they're likely to prefer that and/or not have Chromium
installed.

Also provide `npm run test-unit-…` commands which run unit tests
in a single specific browser only (overrides the conf array).

Follows-up #672.

package.json

@@ -3,6 +3,9 @@
   "scripts": {
     "test": "npm run test-unit-local",
     "test-unit-local": "karma start tests/karma.conf.local.js",
+    "test-unit-firefox": "karma start tests/karma.conf.local.js --browsers FirefoxHeadless",
+    "test-unit-chromium": "karma start tests/karma.conf.local.js --browsers ChromiumHeadless",
+    "test-unit-chrome": "karma start tests/karma.conf.local.js --browsers ChromeHeadless",
     "test-unit-saucelabs": "karma start tests/karma.conf.saucelabs.js"
   },
   "engines": {

tests/karma.conf.local.js

@@ -4,7 +4,13 @@ module.exports = function (config) {
     // https://karma-runner.github.io/5.2/config/browsers.html
     browsers: [
       'FirefoxHeadless',
-      'ChromeHeadless'
+      // During local development, consider Chrome and Chromium to be similar enough
+      // and pick whichever the developer is most likely to have.
+      // In general, Linux distros provide and update Chromium only,
+      // whereas Windows and macOS users tend to have auto-updating Google Chrome.
+      //
+      // See package.json for commands to run tests in a single browser only.
+      process.platform === 'linux' ? 'ChromiumHeadless' : 'ChromeHeadless'
     ],
     frameworks: ['qunit'],
     client: {