Revert "remove run_tests command, use -d -u modname --test-enable or --test-file"

This reverts commit e5fdb3a060a4cec6ebca27caa8789b8e7d5470cf.
Restore run_tests
Fix detection of MIME type in message_parse()

Be careful, content-type may contain tricky content like in the
following example:

Content-Type: multipart/related;
  boundary="_004_3f1e4da175f349248b8d43cdeb9866f1AMSPR06MB343eurprd06pro_";
  type="text/html"

So test the MIME type with startswith()