pgsql: Another try to fix the test case added by commit f5fc2f5b23.

Another try to fix the test case added by commit f5fc2f5b23.

As per analysis, it appears that the 'drop slot' message from the previous
test and 'create slot' message of the new test are either missed or not
yet delivered to the stats collector due to which we will still see the
stats from the old slot. This can happen rarely which could be the reason
that we are seeing some failures in the buildfarm randomly. To avoid that
we are using a different slot name for the tests in
test_decoding/sql/stats.sql.

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

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/51ef9173030cc196c6633ae82b7b32f404b0f768

Modified Files
--------------
contrib/test_decoding/expected/stats.out | 62 ++++++++++++++++----------------
contrib/test_decoding/sql/stats.sql      | 20 ++++++-----
2 files changed, 43 insertions(+), 39 deletions(-)