TST: Add tests for `repr(pd.Period)`

[smarie]

@MarcoGorelli suggested in #51459 (review) to add more tests for Period repr before #51459 can be merged.

What I'm most concerned about is that not all these reprs seems to be explicitly tested for. Some of them are, e.g.

pandas/pandas/tests/scalar/period/test_period.py

Lines 711 to 719 in 044227f

	def test_millisecond_repr(self):
	p = Period("2000-01-01 12:15:02.123")
	assert repr(p) == "Period('2000-01-01 12:15:02.123', 'L')"
	def test_microsecond_repr(self):
	p = Period("2000-01-01 12:15:02.123567")
	assert repr(p) == "Period('2000-01-01 12:15:02.123567', 'U')"

but then FR_SEC isn't (as far as I can tell).

Reckon we should add such unit tests for each period group (perhaps in a separate PR, which we could get in very quickly)? That would give me much more confidence about merging this