Don't abort() in assertions if we can avoid it

[nemequ]

There are some disadvantages to calling abort() in the assertion macros when not forking… perhaps the most problematic is that, when we're not in a forked child, the contents of stderr will be swallowed since the test runner exits before it can splice it to stderr. Also, the test runner exits and no more tests are run, which is obviously not what we want.

I think the easiest way to address this would be to setjmp with some thread-local storage right before we exec the test case, then longjmp back in the assertion.

[nemequ]

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