More property tests

[meooow25]

Closes #1066.

[meooow25]

CI is failing with

tests/map-properties.hs:2041:1: error: [GHC-29916]
    Multiple declarations of ‘prop_mapKeysWith’
    Declared at: tests/map-properties.hs:1590:1
                 tests/map-properties.hs:2041:1
     |
2041 | prop_mapKeysWith f g m =
     | ^^^^^^^^^^^^^^^^

Error: [Cabal-7125]
Failed to build test:map-strict-properties from containers-tests-0.

I don't understand what's going on... 🤔
There is a single prop_mapKeysWith, and it is on line 1963, not 1590 or 2041.

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Edit: Ah, it was testing a merged-on-master state which does have a duplicate definition. Fixed.