clar: Introduce assertion helpers for threads

Don't `cl_git_pass` in a child thread.  When the assertion fails, clar
will `longjmp` to its error handler, but:

> The effect of a call to longjmp() where initialization of the jmp_buf
> structure was not performed in the calling thread is undefined.

Instead, set up an error context that threads can populate, and the
caller can check.

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