clean up `TestHelper` and `TestHarness`

[jvoigtlaender]

For example, tcWithTimeout as a function name is not very descriptive. Not even telling us that it is specifically for IOTasks.

[jvoigtlaender]

Also it's not very clear to me when a function lives in TestHelper and when in TestHarness. For example, why is syntaxCheck in the latter rather than in the former?