Looks like those failures in test_tools happen only when test_check_c_globals is imported and a test in test_imp is run.

```
$ cat matchfile
test.test_tools.*
test.test_imp.ImportTests.test_load_from_source

$ ./python -m test --matchfile matchfile test_check_c_globals test_imp test_tools
0:00:00 load avg: 0.90 Run tests sequentially
0:00:00 load avg: 0.90 [1/3] test_check_c_globals
0:00:00 load avg: 0.90 [2/3] test_imp -- test_check_c_globals run no tests
0:00:00 load avg: 0.90 [3/3] test_tools
test test_tools failed -- multiple errors occurred; run in verbose mode for details
test_tools failed

== Tests result: FAILURE ==

1 test OK.

1 test failed:
    test_tools

1 test run no tests:
    test_check_c_globals

Total duration: 1.9 sec
Tests result: FAILURE
```

If I remove test_check_c_globals from the second command or test.test_imp.ImportTests.test_load_from_source from matchfile, test_tools does not fail.