[kotlin] Stacktraces from WorkflowTester timeouts are useless

[zach-klippenstein]

Timeout exceptions thrown from the coroutine library's withTimeout function are useless. We should catch and re-throw with a real exception that has a real stacktrace.

[zach-klippenstein]

The comment on this issue makes it sound like this should be fixed already, but apparently it is not: Kotlin/kotlinx.coroutines#1625 (comment)

[zach-klippenstein]

Since it affects which exceptions we expose, I would consider this as an API change even though it's technically not.

[zach-klippenstein]

Not API change, not 1.0 blocker.