[TRACING] Add test which would catch the linked panic

[sarsko]

See #128 (comment).

Resolving this issue would involve creating a test which panics when run under #9a29bca, but does not panic when run under #c4c6517.

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.