Clean up test cases for compute.manager._check_instance_build_time

This patch cleans up the unit tests related to the
_check_instance_build_time method so that they are actually verifying
the expected vm_state value when the instance is updated.

This patch also removes the test_instance_build_timeout test case
since it was completely redundant with the other one modified
here. I checked the git history and both test cases were added
in the same commit which confirms that they weren't added for
different reasons, i.e. separate bug fixes/use cases.

Change-Id: Ib384c84e2954bd303661386256053b3c1ad38239
Merge "Clean up test cases for compute.manager._check_instance_build_time"