REF/TST: misplaced hashtable tests, rename HashTable.map map_keys_to_values
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Not sure if these errors are related to this PR or not, but seeing several CI build failures on the test below:

Examples that have failed this test:
#46163
#45575

```
def test_vector_resize(
        self, writable, htable, uniques, dtype, safely_resizes, nvals
    ):
        # Test for memory errors after internal vector
        # reallocations (GH 71[57](https://github.com/pandas-dev/pandas/runs/5348598022?check_suite_focus=true#step:11:57))
        vals = np.array(np.random.randn(1000), dtype=dtype)
    
        # GH 21[68](https://github.com/pandas-dev/pandas/runs/5348598022?check_suite_focus=true#step:11:68)8 ensures we can deal with read-only memory views
        vals.setflags(write=writable)
    
        # initialise instances; cannot initialise in parametrization,
        # as otherwise external views would be held on the array (which is
        # one of the things this test is checking)
        htable = htable()
        uniques = uniques()
    
        # get_labels may append to uniques
        htable.get_labels(vals[:nvals], uniques, 0, -1)
        # to_array() sets an external_view_exists flag on uniques.
        tmp = uniques.to_array()
        oldshape = tmp.shape
    
        # subsequent get_labels() calls can no longer append to it
        # (except for StringHashTables + ObjectVector)
        if safely_resizes:
            htable.get_labels(vals, uniques, 0, -1)
        else:
            with pytest.raises(ValueError, match="external reference.*"):
>               htable.get_labels(vals, uniques, 0, -1)
E               Failed: DID NOT RAISE <class 'ValueError'>
``` @lukemanley possible. have merge quite a lot of things. happy to take a patch if this is still happening on master.