Fix adding keys to waiting list in tests setup

[mariusmihaic]

• There were some bugs in two testing helper functions which worked with staking queue: SaveOneKeyToWaitingList &AddKeysToWaitingList. If no key were already saved in waiting list, AddKeysToWaitingList would have failed.
• Removed SaveOneKeyToWaitingList
• Refactored AddKeysToWaitingList to work by adding one/multiple keys after multiple calls
• Moved getWaitingListElement from staking v4 integration tests into stakingCommon.go

[codecov]

Codecov Report

Merging #4049 (363865a) into feat/liquid-staking (1464814) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 363865a differs from pull request most recent head a3bbea0. Consider uploading reports for the commit a3bbea0 to get more accurate results

@@                   Coverage Diff                   @@
##           feat/liquid-staking    #4049      +/-   ##
=======================================================
- Coverage                75.38%   75.38%   -0.01%     
=======================================================
  Files                      622      622              
  Lines                    83275    83275              
=======================================================
- Hits                     62779    62774       -5     
- Misses                   15773    15776       +3     
- Partials                  4723     4725       +2     

Impacted Files	Coverage Δ
storage/txcache/txListBySenderMap.go	97.50% <0.00%> (-2.50%)	⬇️
p2p/libp2p/netMessenger.go	74.56% <0.00%> (-0.40%)	⬇️

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