tests: unify fixture() helper with Path constant

Change-Id: I63751042391f5cc3e06af7067bc83d67bd0716dc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/569441
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

git_config.py

@@ -111,7 +111,7 @@ class GitConfig:
         return cls(configfile=os.path.join(gitdir, "config"), defaults=defaults)
 
     def __init__(self, configfile, defaults=None, jsonFile=None):
-        self.file = configfile
+        self.file = str(configfile)
         self.defaults = defaults
         self._cache_dict = None
         self._section_dict = None