Stabilise gen_regexp again

While investigating a 5% runtime regression, I realised that
`gen_regexp` wasn't sufficiently stable wrt. GC parameterisations.

This is now rectified by iterating 500 instead of 100 times.
1 job for master in 5 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #12363
validate

00:05:35