Improves test coverage of init-sync queue

[farazdagi]

What type of PR is this?

Other / Better Tests

What does this PR do? Why is it needed?

• We need to make sure that FSMs state transition functions are tested completely (for all valid input states, output states, possible execution routes). Otherwise, it is hard to guarantee correct execution of state machine functioning when we need to hot patch init-sync (we had to do that on the start of Medalla).
• This PR aims for a full coverage of state transition functions (and the rest of the queue component, if there will some untested areas left).

Which issues(s) does this PR fix?

N/A

Other notes for review

[codecov]

Codecov Report

Merging #7320 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7320      +/-   ##
==========================================
- Coverage   60.08%   59.99%   -0.10%     
==========================================
  Files         416      416              
  Lines       30017    29888     -129     
==========================================
- Hits        18037    17930     -107     
- Misses       9024     9028       +4     
+ Partials     2956     2930      -26     

[nisdas]

These are great test cases 🥇