eval: split TestTransactionGroup from BlockEvaluator using TransactionGroupTester

[cce]

Summary

eval.TestTransactionGroup (and TestTransaction) need only read-only access to a few details about the next pending block. This introduces an interface TestEvalContext that is used by the transaction pool, so that there can be parallel TX backlog workers checking if transactions are well-formed and move them from the backlogQueue to the postVerificationQueue.

Test Plan

Updated TxHandler test that was checking Remember() vs Test() and counting metrics bumped by different kinds of err returns.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 56.20%. Comparing base (3b3b5ce) to head (24bb98b).
Report is 26 commits behind head on master.

Files with missing lines	Patch %	Lines
data/pools/transactionPool.go	55.55%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5818      +/-   ##
==========================================
+ Coverage   54.28%   56.20%   +1.91%     
==========================================
  Files         494      494              
  Lines       69936    69958      +22     
==========================================
+ Hits        37968    39318    +1350     
+ Misses      29246    27962    -1284     
+ Partials     2722     2678      -44     

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[jannotti]

Seems like a good idea, I had small comments, but willing to approve as is.