Improve stability of new CSC eviction test (#8160)
Improve stability of new CSC eviction test (#8160)

c4fdf09c0 added a test that now fails with valgrind
it fails for two resons:
1) the test samples the used memory and then limits the maxmemory to
   that value, but it turns out this is not atomic and on slow machines
   the background cron process that clean out old query buffers reduces
   the memory so that the setting doesn't cause eviction.
2) the dbsize was tested late, after reading some invalidation messages
   by that time more and more keys got evicted, partially draining the
   db. this is not the focus of this fix (still a known limitation)
antirez/redis,neomantra/redis,charsyam/redis,neomantra/redis,neomantra/redis,antirez/redis,yossigo/redis,PKRoma/redis,PKRoma/redis,soloestoy/redis,neomantra/redis,yossigo/redis,oranagra/redis,yossigo/redis,ofirluzon/redis,oranagra/redis,ofirluzon/redis,charsyam/redis,soloestoy/redis,PKRoma/redis,charsyam/redis,yossigo/redis,PKRoma/redis,ofirluzon/redis,neomantra/redis,oranagra/redis,PKRoma/redis,soloestoy/redis,soloestoy/redis,ofirluzon/redis,oranagra/redis,antirez/redis,yossigo/redis,ctripcorp/redis,ctripcorp/redis,ctripcorp/redis,charsyam/redis,ctripcorp/redis,oranagra/redis,soloestoy/redis,ofirluzon/redis,charsyam/redis,antirez/redis