Improve usability of `testForkGrouping` and enable it in Mill codebase #982

Job	Run time
test-docs	9m 36s
build-linux / run	3m 41s
build-windows / run	5m 19s
lint-autofix / run	2m 23s
compiler-bridge / run	1m 7s
linux (11, '{scalajslib,scalanativelib,kotlinlib}.__.testCached') / run	17m 0s
linux (11, 'example.{depth,extending}.__.local.testCached') / run	13m 54s
linux (11, 'example.thirdparty[{mockito,acyclic,commons-io}].local.testCached') / run	4m 33s
linux (11, 'integration.{failure,feature,ide}.__.local.testCached') / run	12m 10s
linux (17, '{main,scalalib,testrunner,bsp,testkit}.__.testCached') / run	11m 7s
linux (17, 'example.javalib.__.local.testCached') / run	10m 38s
linux (17, 'example.kotlinlib.__.local.testCached') / run	14m 51s
linux (17, 'example.scalalib.__.local.testCached') / run	15m 17s
itest (11, ci/test-mill-dev.sh && ci/test-mill-release.sh && ./mill -i -k __.ivyDepsTree && ./mil... / run	3m 35s
itest (17, ci/test-mill-bootstrap.sh) / run	9m 47s
linux (17, 'example.thirdparty[{fansi,jimfs,netty,gatling}].local.testCached') / run	6m 26s
linux (17, 'integration.invalidation.__.server.testCached') / run	9m 18s
linux (17, 'integration.invalidation.__.fork.testCached') / run	12m 35s
linux (17, contrib.__.testCached) / run	4m 15s
windows (11, 'integration.invalidation[_].server.testCached') / run	7m 41s
windows (11, contrib.__.testCached) / run	7m 25s
windows (11, "{main,scalalib,bsp}.__.testCached") / run	7m 25s
windows (11, "example.scalalib.{basic,web}.__.fork.testCached") / run	7m 48s
windows (17, 'integration.{feature,failure}[_].fork.testCached') / run	7m 13s
	3h 25m 4s

Provide feedback