test: enable TxQ unit tests work with variable reference fee (#5118)

[vvysokikh1]

High Level Overview of Change

Fix TxQ unit tests to be able to process reference fee value other than 10.

Context of Change

In preparation for potential reference fee change we would like to verify that fee change works as expected. The first step is to fix all unit tests to be able to work with different reference fee values.

Type of Change

• [x ] Tests (you added tests for code that already exists, or your new feature included in this PR)

API Impact

None

Test Plan

Tested following reference fee values: 10, 20, 50, 100, 200, 500, 1000 by changing https://github.com/vvysokikh1/rippled/blob/2f432e812cb773048530ebfaf2e0e6def51e3cc2/src/test/jtx/impl/envconfig.cpp#L47

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.2%. Comparing base (1fbf8da) to head (2f14074).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5118     +/-   ##
=========================================
- Coverage     76.2%   76.2%   -0.0%     
=========================================
  Files          760     760             
  Lines        61568   61568             
  Branches      8126    8122      -4     
=========================================
- Hits         46909   46905      -4     
- Misses       14659   14663      +4     

see 3 files with indirect coverage changes