Fix expression tests

[Fokko]

test_not_equal_to_invert should test unbound References,

Bound one are tested here:

iceberg-python/tests/expressions/test_expressions.py

Lines 363 to 373 in d99936a

	def test_bound_not_equal_to_invert(table_schema_simple: Schema) -> None:
	bound = BoundNotEqualTo(
	BoundReference(table_schema_simple.find_field(1), table_schema_simple.accessor_for_field(1)), literal("hello")
	)
	assert ~bound == BoundEqualTo(
	term=BoundReference(
	field=NestedField(field_id=1, name="foo", field_type=StringType(), required=False),
	accessor=Accessor(position=0, inner=None),
	),
	literal=literal("hello"),
	)

Rationale for this change

Are these changes tested?

Are there any user-facing changes?

[kevinjqliu]

LGTM!