fix test function discovery regexp

[adakraz]

Currently code looks for functions with the word "test" anywhere in its name.
This commit makes is_test_key accept functions starting with optional underscores and the word "test".

This code introduces backwards incompatibility, but I suppose uncommon formats are not (ab)used too much by anyone.

[adakraz]

FYI, it is a "hook" now, as is_test_key is not local. ;)