TST: move DataFrame.applymap tests to tests/frame/methods/test_map.py
Currently the tests for `DataFrame.applymap` are mixed together with the tests for `DataFrame.apply`. Keeping them separate makes it easier to find tests for one method so I've moved the `applymap` ones to tests/frame/methods/test_map.py.

Note there is also a proposal to rename `DataFrame.applymap` to `DataFrame.map` (#52364). The one who gets merged later of this PR or 52364 will need to be rebased.

xref #52384.
Thanks @topper-123 