Some MTR tests fail when run with --ssl option

Some seemingly unrelated tests fail when they are run with --ssl option. It shows two problems:

• the tests that fail with --ssl should be either skipped (if they can't be run this way due to conflicting configuration), or updated to avoid mismatches caused by ssl;
• we don't have an --ssl run in buildbot. It should probably be added when the tests are fixed.

See also MDEV-7697 which shows a real problem with SSL which a test run with --ssl would have caught.

Examples:
main.ssl_crl_clients
perfschema.nesting