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
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2