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