CI/TST: Fix xfail(strict=False) condition

[mroeschke]

• xref CI/TST: Flaky test test_union_different_types #46144
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.

cc @jbrockmendel For this example, the monotonic condition was too strict

In [1]: index_flat = Index([True, True, True, True, True, True, True, True, False, False], dtype='bool')

In [2]: index_flat2 = pd.Int64Index([0, 0, 1, 1, 2, 2], dtype='int64')
<ipython-input-2-0cd74eb77800>:1: FutureWarning: pandas.Int64Index is deprecated and will be removed from pandas in a future version. Use pandas.Index with the appropriate dtype instead.
  index_flat2 = pd.Int64Index([0, 0, 1, 1, 2, 2], dtype='int64')

In [3]: idx1 = index_flat

In [4]: idx2 = index_flat2

In [5]: (
   ...:             not idx1.is_unique
   ...:             and not idx2.is_unique
   ...:             and not idx2.is_monotonic_decreasing
   ...:             and idx1.dtype.kind == "i"
   ...:             and idx2.dtype.kind == "b"
   ...:         )
Out[5]: False

In [6]: (
   ...:             not idx2.is_unique
   ...:             and not idx1.is_unique
   ...:             and not idx1.is_monotonic_decreasing
   ...:             and idx2.dtype.kind == "i"
   ...:             and idx1.dtype.kind == "b"
   ...:         )
Out[6]: False

[jbrockmendel]

could add a comment pointing back here to the effect of "not idx2.is_monotonic_decreasing covers most cases but is too strict"

[jbrockmendel]

LGTM