Introduce 'norace' build tag that is used to
filter out test files containing test cases
or test suites that, if run with the -race
flag on would cause race conditions.
Supersede #7300.
Prepare migrating testing auxiliary functions from tests
to testutil.
Remove local duplicates on testutil.WriteToNewTempFile().
Always favor testutil.NewTestCaseDir() over ioutil.TempDir().
Add test cases for the testing auxiliary functions.