Investigate JS test failures that are silent on TeamCity

[qwwdfsad]

Steps to reproduce:
./gradlew :kotlinx-coroutines-core:jsIrTest

Output, a lot of:

Too many unhandled exceptions 1, expected 0, got: CompletionHandlerException: Exception in completion handler InvokeOnCompletion@17[job@18] for BroadcastCoroutine{Cancelled}@18: CompletionHandlerException: Exception in completion handler InvokeOnCompletion@17[job@18] for BroadcastCoroutine{Cancelled}@18
CompletionHandlerException: Exception in completion handler InvokeOnCompletion@17[job@18] for BroadcastCoroutine{Cancelled}@18