Add function to pump IPC process until string match

Add function to pump IPC process until string match

Refactor the recovery tests to not carry a local duplicated copy of
the pump_until function which pumps a process until a defined string
is seen on a stream. This reduces duplication, and is in preparation
for another patch which will also use this functionality.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion https://postgr.es/m/YgynUafCyIu3jIhC@paquier.xyz