If a Tor unit test causes a BUG log, it should fail

In legacy/trac#28660 (moved), a successful unit test logged a BUG warning, but the test still succeeded.

We should make tests fail by default if they log a BUG() or LD_BUG warning, but allow some tests to expect bug warnings.

I thought we made this change, or had a ticket for this change, but I couldn't find it.

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