[lldb/Test] Bypass LLDB_TEST_COMMON_ARGS for certain dotest args (NFC)

Rather than serializing every argument through LLDB_TEST_COMMON_ARGS, we
can pass some of them directly using their CMake variable. Although this
does introduce some code duplication between lit's site config and the
lldb-dotest utility, it also means that it becomes easier to override
these values (WIP).