[TRACING] Add test which would catch the linked panic

See https://github.com/awslabs/shuttle/pull/128#issuecomment-1830384770.

Resolving this issue would involve creating a test which panics when run under [#9a29bca](https://github.com/awslabs/shuttle/pull/128/commits/9a29bcac1d30fee1e2189a398edd9e70d86ac617), but does not panic when run under [#c4c6517](https://github.com/awslabs/shuttle/pull/128/commits/c4c651764e106de357ab2a6f8cd6350d3dd0a508).

The issue seems to have been:
1. Task X enters Span A. Context-switch to Task Y.
2. Task Y enters Span A. Task Y leaves Span A, which closes it. Context switch to Task X.
3. Task X tries to enter Span A, which does not exist anymore.

May be more details needed, have not looked into this issue much.

Start with the linked pull request comment and compare commits 9a29bca and c4c6517. Reproduce the Task X/Task Y Span A sequence described in the issue, then add a regression test that panics under the first commit but not the second; done means the test captures the linked tracing panic.

Written by the indexing model from the issue text.