QAR-1619: Finish replacing RunBuildTest.cmake with run_build_test.py.

Because the details of RunBuildTest.cmake versus run_build_test.py had to be
changed in so many different places, introduce LL_TEST_COMMAND CMake macro (in
LLTestCommand.cmake) to encapsulate construction of the actual command line.
Use LL_TEST_COMMAND in LL_ADD_PROJECT_UNIT_TESTS, LL_ADD_INTEGRATION_TEST, the
big indra/test monolith and the various LslCompilerMacros.
Fix run_build_test.py to pass through the test executable's own options (e.g.
--touch, --output) without inspection. Defend it against the case when the
platform-specific library path environment variable doesn't yet exist. Make it
report errors only on nonzero test-program rc.
Remove RunBuildTest.cmake.