Feat/counter merge operator tests

[devyetii]

Fixes

• Fixing simple issue in counter merge operator
• Adding unit tests to the merge operator

Changes

Need to be mentioned in CHANGELOG.md?

Tests

Tasks to complete before merging PR:

• Ensure system tests are passing. If not Run them manually to check for any regressions 📋
• Do any new system tests need added to test this change? do any existing system tests need updated? If so create a PR at 0chain/system_test
• Merge your system tests PR to master AFTER merging this PR

Associated PRs (Link as appropriate):

• blobber:
• gosdk:
• system_test:
• zboxcli:
• zwalletcli:
• Other: ...

[codecov-commenter]

Codecov Report

Merging #2508 (697348d) into staging (47efd9b) will increase coverage by 0.01%.
The diff coverage is 100.00%.

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@@             Coverage Diff             @@
##           staging    #2508      +/-   ##
===========================================
+ Coverage    28.78%   28.80%   +0.01%     
===========================================
  Files          380      380              
  Lines        63877    63877              
===========================================
+ Hits         18385    18397      +12     
+ Misses       43325    43311      -14     
- Partials      2167     2169       +2     

Flag	Coverage Δ
Unit-Tests	28.80% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...in.net/core/ememorystore/counter_merge_operator.go	50.00% <100.00%> (+50.00%)	⬆️

... and 4 files with indirect coverage changes

[peterlimg]

@devyetii looks good to me, please also check the err here, we should not ignore the error.

[peterlimg]

I'll merge and fix error checking here in a new PR.