Feature request: different error messages for hanging test body vs launched coroutines

[joffrey-bion]

From my experience testing a coroutine-based library, with tons of fine-tuned suspension assertions, it has often been important to me to distinguish cases where the test body itself hangs from cases where some coroutines (spawned from deep in the code) are hanging.

When running tests locally it's easy to know by debugging tests and putting a breakpoint at the end of the test method, but when running on CI this is not an option. Sometimes I can't even reproduce test failures locally (because I'm developing on Windows, and the failing test happens in iOS test code on MacOS CI agent).

It would be nice if this error message could make the distinction too:

The test coroutine was not completed after waiting for 5000 ms
kotlinx.coroutines.test.UncompletedCoroutinesError: The test coroutine was not completed after waiting for 5000 ms
	at app//kotlinx.coroutines.test.TestBuildersKt__TestBuildersKt$runTestCoroutine$3$3.invokeSuspend(TestBuilders.kt:231)
	(Coroutine boundary)
	at kotlinx.coroutines.test.TestBuildersKt__TestBuildersKt.runTestCoroutine(TestBuilders.kt:257)

Currently I get the same error for both of these test cases:

@Test
fun testTimeoutWithHangingBody() = runTest(dispatchTimeoutMs = 5000) {
    awaitCancellation() // hanging test body
}
    
@Test
fun testTimeoutWithHangingCoroutine() = runTest(dispatchTimeoutMs = 5000) {
    launch {
        awaitCancellation() // hanging launched coroutine
    }
}

[qwwdfsad]

Orthogonally to this issue, it can be helpful to run the tests with DebugProbes enabled.
Potentially, we can even integrate with them out-of-the-box and dump all coroutines (or only tests-related) if the probes are enabled