run-tests: use context managers for file descriptors I've seen the following error a few times recently when running the tests with `yes | ./run-tests.py --local -j9 -i`: Errored test-add.t: Traceback (most recent call last): File "./run-tests.py", line 821, in run self.runTest() File "./run-tests.py", line 910, in runTest if self._result.addOutputMismatch(self, ret, out, self._refout): File "./run-tests.py", line 1774, in addOutputMismatch rename(test.errpath, test.path) File "./run-tests.py", line 571, in rename os.remove(src) WindowsError: [Error 32] The process cannot access the file because it is being used by another process: 'c:\\Users\\Matt\\projects\\hg\\tests\\test-add.t.err' This change doesn't fix the problem, but it seems like a simple enough improvement.