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

tests/test-check-code.t

@@ -90,6 +90,6 b' Keep python3 tests sorted:'
 
 Keep Windows line endings in check
 
-  $ hg files 'set:eol(dos)'
+  $ testrepohg files 'set:eol(dos)'
   contrib/win32/hg.bat
   contrib/win32/mercurial.ini