Fix fail in test_repo_trace if trace is disabled

If env has REPO_TRACE 0, then the test produces:
FileNotFoundError: No such file or directory: TRACE_FILE_from_test'
because it assumes trace is always enabled.

Improve by enabling trace for this test class.

Also modify the repo_trace.SetTrace API to take an optional
True/False if needed, still defaults to True though.

Change-Id: I2bb4632eb45ee529fc1785833e0bafd9c36f0875
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/445541
Commit-Queue: Fredrik de Groot <fredrik.de.groot@haleytek.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Brian Gan <brgan@google.com>
Tested-by: Fredrik de Groot <fredrik.de.groot@haleytek.com>

tests/conftest.py

@@ -26,6 +26,7 @@ import repo_trace
 @pytest.fixture(autouse=True)
 def disable_repo_trace(tmp_path):
     """Set an environment marker to relax certain strict checks for test code."""  # noqa: E501
+    repo_trace.SetTrace(False)
     repo_trace._TRACE_FILE = str(tmp_path / "TRACE_FILE_from_test")