Rollup merge of #154761 - Vastargazing:add-regression-tests-cmp-argument-order, r=jhpratt

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

PR rust-lang/rust#136307 introduced a regression in min_by, max_by, and minmax_by:
the compare closure received arguments as (v2, v1) instead of (v1, v2),
contrary to the documented contract.
Although this was fixed in rust-lang/rust#139357, no regression tests were added.
This PR adds regression tests for all three functions, verifying that compare
always receives arguments in the documented order (v1, v2).
As a bonus: first coretests coverage for minmax_by.