Tests should explicitly check for schema_id

[kevinjqliu]

This PR modifies tests to explicitly check schema_id when comparing Schema.
Related to #290

Previously, many tests were written with implicit schema_id checks. For example,

    assert table.schema() == Schema(
        NestedField(field_id=1, name='id', field_type=LongType(), required=False),
        NestedField(field_id=2, name='event_ts', field_type=TimestampType(), required=False),
        NestedField(field_id=3, name='str', field_type=StringType(), required=False),
        NestedField(field_id=4, name='col_string', field_type=StringType(), required=False),
        schema_id=1,
        identifier_field_ids=[],
    )

Since Schema.__eq__ does not check for schema_id, we never check the schema_id of the Schema

[Fokko]

Thanks for updating the tests @kevinjqliu This would have caught the bug in #290 earlier 👍