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>

tests/utils_for_test.py

@@ -27,6 +27,11 @@ from typing import Optional, Union
 import git_command
 
 
+THIS_FILE = Path(__file__).resolve()
+THIS_DIR = THIS_FILE.parent
+FIXTURES_DIR = THIS_DIR / "fixtures"
+
+
 def init_git_tree(
     path: Union[str, Path],
     ref_format: Optional[str] = None,