yes, once_cell better, but ...
this reduces from
==31349== Total: 1,365,199,543 bytes in 4,774,213 blocks
==31349== At t-gmax: 10,975,708 bytes in 66,093 blocks
==31349== At t-end: 2,880,947 bytes in 12,332 blocks
==31349== Reads: 5,210,008,956 bytes
==31349== Writes: 1,280,920,127 bytes
to
==47796== Total: 821,467,407 bytes in 3,955,595 blocks
==47796== At t-gmax: 10,976,209 bytes in 66,100 blocks
==47796== At t-end: 2,944,016 bytes in 12,490 blocks
==47796== Reads: 4,788,959,023 bytes
==47796== Writes: 975,493,639 bytes
miropt-test-tools: remove regex usage
this removes regex usage and slightly refactors ext stripping in one case
