Fix flaky tagged-PDF skip-text test under Ghostscript 10.x

The test asserted that --mode skip preserves the structure tree, but with
the default --output-type auto the output runs through Ghostscript PDF/A
conversion, which discards /StructTreeRoot on Ghostscript 10.x (9.x kept
it). This failed on macOS CI and locally while passing on the Ubuntu
runners' Ghostscript 9.55. Pin the test to --output-type pdf so it exercises
OCRmyPDF's own structure-tree handling without the version-dependent GS step,
and document the caveat in advanced.md.