c186ba13 has fixed an issue related to the updates of the minimum
recovery LSN across multiple processes on standbys, but we never really
had a test case able to reliably check its logic.

This commit introduces a new test case to close the gap, and is designed
to check the consistency of data based on the minimum recovery point set
by either the startup process or the checkpointer for both an offline
cluster (by looking at the on-disk page headers) and an online cluster
(using pageinspect).

Note that with c186ba13 reverted, this test fails badly for both the
online and offline cases, as designed.

Author: Michael Paquier, Andrew Gierth
Reviewed-by: Andrew Gierth, Georgios Kokolatos, Arthur Zakirov
Discussion: https://postgr.es/m/20181108044525.GA17482@paquier.xyz