Add test cases to improve mutation score indicator

[Kourin1996]

Description

Add test cases in message package to improve MSI (mutation score indicator)

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Breaking changes

Please complete this section if any breaking changes have been made, otherwise delete it

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have added sufficient documentation in code

Additional comments

Please post additional comments in this section if you have them, otherwise delete it

[codecov]

Codecov Report

Merging #60 (dda3271) into mutation-tests-only (da4fb4f) will decrease coverage by 2.30%.
The diff coverage is 58.82%.

❗ Current head dda3271 differs from pull request most recent head 97bd746. Consider uploading reports for the commit 97bd746 to get more accurate results

@@                   Coverage Diff                   @@
##           mutation-tests-only      #60      +/-   ##
=======================================================
- Coverage                95.51%   93.20%   -2.31%     
=======================================================
  Files                        6        6              
  Lines                     1292     1339      +47     
=======================================================
+ Hits                      1234     1248      +14     
- Misses                      42       73      +31     
- Partials                    16       18       +2     

Impacted Files	Coverage Δ
messages/helpers.go	88.97% <18.75%> (-11.03%)	⬇️
core/ibft.go	91.39% <69.38%> (-3.68%)	⬇️
messages/event_manager.go	100.00% <100.00%> (ø)
messages/messages.go	97.20% <100.00%> (+5.58%)	⬆️
messages/event_subscription.go	94.59% <0.00%> (+8.10%)	⬆️

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[JekaMas]

What is the result score?

[Kourin1996]

@JekaMas thanks for reviewing. make mut told the score was 0.989011. But this score is sometimes different every time I ran mutation test, I will run mutation test 10 times. And will finish fix of lint issue today.

[Kourin1996]

@JekaMas I have one quick question. I've not seen any issue from core package. Does it mean there is no issue in core package or we're skipping test for core for now?

[Kourin1996]

I ran test 10 times and got the following scores:

MSI: "0.9862637362637363"
MSI: "0.978021978021978"
MSI: "0.9697802197802198"
MSI: "0.967032967032967"
MSI: "0.9642857142857143"
MSI: "0.9642857142857143"
MSI: "0.9642857142857143"
MSI: "0.9642857142857143"
MSI: "0.9642857142857143"
MSI: "0.9642857142857143"

the average is about 0.968