test: rework function tests to have asset unlock functionality activated via mn_rr

[PastaPastaPasta]

see todo introduced here: 1d500f8#diff-e771a3dd26d84715d137f42a9d77361684707ebb905310fc678ada1e47d6112fR59

This should reword the functional test to work while the credit pool is increasing in the background due to mn_rr; then remove the conditional check in validation for regtest special case.

This should be updated before RC.2.

[UdjinM6]

still not fixed https://github.com/dashpay/dash/blob/v20.1.0/src/evo/specialtxman.cpp#L60, moving to v21

[knst]

done. see #6269