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 51.14%. Comparing base (b7b3e5e) to head (d04bda9).
Report is 1 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   51.88%   51.14%   -0.75%     
==========================================
  Files         639      639              
  Lines       85489    85511      +22     
==========================================
- Hits        44359    43736     -623     
- Misses      38320    38909     +589     
- Partials     2810     2866      +56     

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[jannotti]

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