gcc-changelog: Add libstdc++-v3/testsuite to wildcard prefixes

gcc-changelog: Add libstdc++-v3/testsuite to wildcard prefixes

This allows using "testsuite/*" in libstdc++-v3/ChangeLog entries, which
was one of the original motivations for adding wildcard support in the
first place:
https://gcc.gnu.org/pipermail/gcc/2020-June/232719.html

contrib/ChangeLog:

	* gcc-changelog/git_commit.py (wildcard_prefixes): Add libstdc++
	testsuite directory.