RBTools has had two base classes available for unit test suites: an older `rbtools.utils.testbase.RBTestBase`, which contained all the useful helper methods but should have been legacy, and `rbtools.testing.testcase.TestCase`, which contained very little but was intended to be modern. This change consolidates the two. All the useful stuff from `RBTestBase` now lives in `TestCase`, with some unused methods (`reset_cl_args()` and `catch_output()`) removed. `gen_uuid()` was also removed, as it's not a generally-useful method, and was only used in one test, which shouldn't have used it to begin with. `RBTestBase` did set up a temporary home directory and `chdir` to it, which is functionality that wasn't worth removing, but shouldn't necessarily be the default. For now, `TestCase` now includes this ability as an opt-in. The old `RBTestBase` is still around for now, for legacy test suites. Many suites still need to be updated to use `TestCase`. It also opts into the temporary home directory, so that there aren't any regressions.