Adds a test function like add_func, but runs test_func from
a [class@Dex.
Fiber]. The calling thread is given a thread-default [class@Dex.Scheduler] if it does not already have one, allowing tests to use await and related APIs directly.
After test_func completes, the scheduler's main context is iterated until no immediately pending sources remain.
| testpath |
test case path |
| test_func |
test function to execute from a fiber |