coretests: add argument order regression tests for min_by/max_by/minmax_by

A recent regression swapped the argument order passed to the compare
closure in min_by, max_by and minmax_by (compare(&v2, &v1) instead of
compare(&v1, &v2)). This was fixed, but no regression test was added.

Add tests that record the arguments the compare closure receives and
assert they match (v1, v2) — the documented contract.