run_build_test.py already has the capability to set environment variables, and
we may as well direct it to set PYTHON to the running Python interpreter. That
completely eliminates one level of process wrapper.

Recent checkins introduced two different CMake macros SET_TEST_LIST (which
returned a CMake list of PATH directory strings) and SET_TEST_PATH (which
returned a single platform-appropriate PATH string). On Windows, whose
path-separator character is ';', SET_TEST_PATH interacted badly with CMake: in
CMake, a single string containing ';' characters is indistinguishable from a
list of strings.
Eliminate the return-single-string form, redirecting the name SET_TEST_PATH to
the macro that returns a CMake list. Make LL_TEST_COMMAND expect a list value,
prepending each directory string with run_build_test.py's -l switch.

Add ${SHARED_LIB_STAGING_DIR_RELEASE} to LD_LIBRARY_PATH for executing tests:
otherwise Debug-build tests can't find (e.g.) the aprutil DLL, for which we
don't build/package a debug variant.
Leverage discovery that a CMake macro can accept a target variable name
argument for LL_TEST_COMMAND macro.

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.