test: mempool overrides

[rootulp]

I wanted to see what value we use for MaxTxsBytes. Since it's computed and not hard-coded, I wrote a test to see it and verify we don't accidentally revert it.

[codecov-commenter]

Codecov Report

Merging #2823 (bab2c92) into main (7e9fb84) will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2823      +/-   ##
==========================================
+ Coverage   19.44%   19.59%   +0.15%     
==========================================
  Files         143      143              
  Lines       17320    17320              
==========================================
+ Hits         3368     3394      +26     
+ Misses      13633    13607      -26     
  Partials      319      319              

see 1 file with indirect coverage changes

[cmwaters]

We should do these kind of compatibility vector tests in many other cases as well to prevent us from accidentally breaking stuff