tests: clean up repo_trace._TRACE_FILE patching

Patch this automatically for all tests rather than duplicating the
boilerplate in diff testcases.

Change-Id: I391d5c859974cda3d5680d34ede2ce6e9e925838
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/354358
Reviewed-by: Joanna Wang <jojwang@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

tests/test_manifest_xml.py

@@ -23,7 +23,6 @@ import xml.dom.minidom
 
 import error
 import manifest_xml
-import repo_trace
 
 
 # Invalid paths that we don't want in the filesystem.
@@ -94,7 +93,6 @@ class ManifestParseTestCase(unittest.TestCase):
   def setUp(self):
     self.tempdirobj = tempfile.TemporaryDirectory(prefix='repo_tests')
     self.tempdir = self.tempdirobj.name
-    repo_trace._TRACE_FILE = os.path.join(self.tempdir, 'TRACE_FILE_from_test')
     self.repodir = os.path.join(self.tempdir, '.repo')
     self.manifest_dir = os.path.join(self.repodir, 'manifests')
     self.manifest_file = os.path.join(