Add unit tests to increase coverage

This patch proposes adding several new unit tests in effort to increase
the total unit test coverage.

Additionally, the ./tempest/serial_tests/ directory is not included in
coverage calculation. The reason being we don't want to test tests with
unit tests, which becomes a bit of an overkill.

Finally, there is an addition of a new option for the coverage program
called --fail-under. It will result the program in failure if the
coverage is under set percentage. Reason for this implementation is to
encourage writing unit tests for new code.

Change-Id: I804116413cd7d73cd7e5ae71409a8855ef937b88
5 files changed