update mqtt clean session test

[suchen-sci]

the TestCleanSession is used to check cleanSession flag in MQTT. If not cleanSession, storage will store session and when client reconnect, it will see previous subscribe topics. Yesterday the test failed (very rare, less than 1/200 runs), the message say client don't see previous subscribed topic when cleanSession is set false.
After careful think about code, I think the reason maybe the goroutine to store session run after we check subscribe topics. To solve this, we first make sure session have been stored in storage, then do check.

[codecov-commenter]

Codecov Report

Merging #396 (4f69044) into main (262061a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #396   +/-   ##
=======================================
  Coverage   80.82%   80.82%           
=======================================
  Files          60       60           
  Lines        6966     6966           
=======================================
  Hits         5630     5630           
+ Misses       1045     1044    -1     
- Partials      291      292    +1     

Impacted Files	Coverage Δ
pkg/cluster/cluster.go	50.59% <0.00%> (ø)

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