As a follow on to D96282, since bug point passes is built as a module the proper file extension to use is LLVM_PLUGIN_EXT, rather than SHLIBEXT. Using SHLIBEXT causes the tests to load a non-existent file on AIX. We also adjust the PluginsTest unittest to use LLVM_PLUGIN_EXT for similar reasons.
This change should hopefully make little difference to other platforms, since generally SHLIBEXT=LTDL_SHLIB_EXT=CMAKE_SHARED_LIBRARY_SUFFIX and LLVM_PLUGIN_EXT=CMAKE_SHARED_LIBRARY_SUFFIX on every platform except AIX.