EXT-7926: fix broken LD_LIBRARY_PATH handling on Windows.

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.