* test/lisp/mh-e/mh-utils-tests.el: Environment variable TEST_MH_PATH controls which installed MH variant to test with. Moved the commentary about testing with different MH variants from above 'with-mh-test-env' definition to "Commentary" section at the top of the file. * test/lisp/mh-e/test-all-mh-variants.sh: New script to test all installed MH variants.