validate: fix bug monitor subscriptions on pads by name

gst_validate_override_register_by_name() was not working when using a
pad name because by the time gst_validate_pad_monitor_do_setup()
was called to set the name of the monitor it was too late for overrides
to have any effect.

Patch written by Thibault.
2 jobs for validate-pad-name-bug in 2 minutes and 50 seconds (queued for 2 seconds)
latest
Status Name Job ID Coverage
  Manifest
passed manifest #44036

00:00:21

 
  Build
passed build fedora x86_64 #44037

00:02:28