The subroutine pump_until provides the functionality to poll until the
given string is matched, or a timeout occurs.  This can be used from other
places as well, so moving it to TestLib.pm.  The immediate need is for an
upcoming regression test patch for dropdb utility.

Author: Vignesh C
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CAP_rwwmLJJbn70vLOZFpxGw3XD7nLB_7+NKz46H5EOO2k5H7OQ@mail.gmail.com