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.

Unit tests pass.