Fix log store ut

[sanebay]

Issue
The rollback was randomly failing because after truncation it expected no rollback records. But the test was writing to only one logstore. Because of other test cases in same file, other log store also have some log entries. Here truncate will truncate all logstores but device truncate will pick a logstore with the least idx and that value was smaller than the rollback records. This is because init was writing to only 1 store.

Fix.
Write to all logstores equally. Write rollback to one of the logstore(logstore 1). Truncate on all and should make all the logstores move forward together and delete those rollback records

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e073fe2) 47.65% compared to head (8f6e186) 58.60%.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #304       +/-   ##
===========================================
+ Coverage   47.65%   58.60%   +10.95%     
===========================================
  Files         107      107               
  Lines        9271     9202       -69     
  Branches     1197     1174       -23     
===========================================
+ Hits         4418     5393      +975     
+ Misses       4390     3290     -1100     
- Partials      463      519       +56     

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