[2/n] test(deposits): EIP-4881 spec tests

[calbera]

• Tested against official spec test cases.
• Use common.Root for consistency of HashTreeRoot() common.Root interface.
• This diff is NOT consensus breaking to boonet.

[codecov]

Codecov Report

Attention: Patch coverage is 65.38462% with 18 lines in your changes missing coverage. Please review.

Project coverage is 31.83%. Comparing base (5bb9a9b) to head (76c153d).

Files with missing lines	Patch %	Lines
storage/deposit/merkle/snapshot.go	52.63%	6 Missing and 3 partials ⚠️
storage/deposit/merkle/tree.go	61.11%	7 Missing ⚠️
storage/deposit/merkle/node.go	84.61%	2 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           fix-deps-htr2    #2277      +/-   ##
=================================================
+ Coverage          31.49%   31.83%   +0.34%     
=================================================
  Files                337      337              
  Lines              15757    15772      +15     
  Branches              20       20              
=================================================
+ Hits                4962     5021      +59     
+ Misses             10445    10394      -51     
- Partials             350      357       +7     

Files with missing lines	Coverage Δ
storage/deposit/merkle/helpers.go	79.72% <100.00%> (+28.37%)	⬆️
storage/deposit/merkle/node.go	49.65% <84.61%> (+11.18%)	⬆️
storage/deposit/merkle/tree.go	58.76% <61.11%> (+13.40%)	⬆️
storage/deposit/merkle/snapshot.go	73.01% <52.63%> (-4.07%)	⬇️