Improve WaitForTestCase

The unit test for rally.benchmark.utils.wait_for tended to be of the longest
continuance. In this patch, we:
* reduce the check interval so that the unit tests run faster;
* split the code into 3 test cases.

Change-Id: Ib2bf5e1fbc2127b6d07d76c8aba5e4bfff64dede
Partial-Bug: #1271831