The unit tests for the rbt command weren't always running the correct

version of RBTools. The main test suite was running whatever rbt was

in the path with whatever version of Python it was installed with, which

didn't guarantee correct results, while the --version tests ran with

the current Python but also ran the first build found in the path.
These could produce wrong results, and in fact have in my own test

environment. To fix this, we now run the bundled copy specifically, and

we now always do so with the current version of Python. This ensures a

reliable test run, and also allows us to consolidate the two test suites

for this command.

Unit tests pass.