Don't write test snapshot if no thread name

[brson]

What

When dropping Env, if there is no thread name, don't write a snapshot file (and don't panic).

Why

The current code for writing test snapshots under testutils assumes the Env has been constructed as part of a unit test, but this is not the case for e.g. fuzz tests.

A fuzz test runs on the main thread and does not configure a thread name, but the test snapshot writer panics if there is no thread name.

This patch adds a (very crude) heuristic to decide that it is not running as part of a unit test and it is ok to not write the test snapshot. I looked for more reliable ways to determine if code was running as part of a unit test but did not find one.

Known limitations

This heuristic may be inaccurate for other unanticipated scenarios.

[leighmcculloch]

Thanks for making this change. This is better. 👍