Add unit tests

[DimaStebaev]

• fix bugs
  • don't delete required nodes history Necessary node history is deleted during clearHistory call #70
  • fix rounding error when schain is paid too late
  • revert on payment for 0 months
  • https://github.com/skalenetwork/internal-support/issues/930
• add tests
  • should not add validator with the same id
  • should not add validator with the same address
  • should not allow to top up more than max replenishment period
  • should set version
  • should allow to blacklist nodes
  • should allow to get information about validators and schains
  • should allow admin to claim rewards for the validator
  • should revert on incorrect time interval
  • should not allow to clean unprocessed data
  • should not allow to add to the processed part
  • should get value
  • should get value of empty sequence
  • should clear values
  • big random test
• improve tests
  • should remove validator
• refactor
  • remove long lines

Resolves #64
Resolves #70

[codecov]

Codecov Report

Attention: Patch coverage is 94.18605% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 95.85%. Comparing base (66251ad) to head (06dfeb9).

Files	Patch %	Lines
contracts/Paymaster.sol	88.46%	3 Missing ⚠️
contracts/types/Skl.sol	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #67      +/-   ##
===========================================
+ Coverage    90.34%   95.85%   +5.50%     
===========================================
  Files           16       17       +1     
  Lines          777      796      +19     
  Branches       115      115              
===========================================
+ Hits           702      763      +61     
+ Misses          75       33      -42     

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[payvint]

Nice!!!