tests: more consistent checks about invalid options * tests/aclocal.test: Grepping of automake stderr for messages reporting invalid options made stricter. * tests/no-outdir-option.test: Likewise. Also, create a dummy `Makefile.am', to ensure that the automake failures are really caused only by unrecognized options. * tests/automake.test: Added trailing `:' command. Removed redundant checks on `--help' and `--version' option (already performed in the test `help*.test').