[Patch, PR 61720] Clear regex BFS match queue after every iteration

[Tim Shen]

This bug shouldn't be introduced, if I wrote clearer code last summer.
Sorry :(. The _Executor is a little bit messy. The _M_match_queue is
not cleared because it's treated `globally` between iterations (in
_M_main_dispatch(..., __bfs)).

Add a new file general_testcases.cc; we can gradually move test cases
to this single file to reduce duplicated compilation in testing?


-- 
Regards,
Tim Shen