Corrected check for libtiff feature

[radarhere]

Follow up to #7975

• I missed a place where WRITE_LIBTIFF is assigned directly in the tests, rather than using monkeypatch instead.
• In parametrizing tests, I forgot that libtiff_support_custom_tags is only present if libtiff support is present. While @skip_unless_feature("libtiff") is there on this test file, the parametrize conditions are still evaluated, so if libtiff support is absent, this causes an error. I've used getattr to fix this.