Fix tests for replication slots stats.

Fix tests for replication slots stats.

Some of the tests were not considering that the slot's spill stats could be
received by the stats collector after we have reset the stats. Remove
those tests and don't check total bytes decoded and sent to output plugin
in the spilled stats test as we can send the spilled stats to the stats
collector before actually sending the changes to output plugin.

Reported-by: Tom Lane as per buildfarm
Author: Vignesh C, Sawada Masahiko
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/20210319185247.ldebgpdaxsowiflw@alap3.anarazel.de