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.
* add a bunch of tests, add DONTCOVER text tag
- Also fix flaky test (closes: #3559). Don't test values
returned by queries since there's no way to query a
specific height via REST.
* GetTempDir -> NewTestCaseDir