TST: add test with freq=2M for period_array from datetime64
xref #52064

added a test in `pandas/tests/arrays/period/test_constructors.py`, which covers a case `freq='2M'` for period_array from datetime64. The reason: existing tests were all for `"freq='M'"`,  rather then for `"freq='2M'"`.
@MarcoGorelli, could you please take a look at this pr? Seems like ci failures aren't related to my changes.