tests: use testrepohg in one more place in test-check-code.t
This is already used elsewhere in the test to access the current hg repo, and
avoids an error about the unknown revlog-compression-zstd when C extensions
aren't built. The only other such error is in test-check-interfaces.py, but I
don't see a way to avoid it other than to create an empty scratch repo.
Differential Revision: https://phab.mercurial-scm.org/D9364