Test: cts-schemas: Implement only one test per upgrade pipeline

Instead of testing each transform within the pipeline, run all tests for
the entire pipeline. This is equivalent to moving all tests to
test2to3roundtrip, except that we now drop the roundtrip stylesheet,
which is not used by Pacemaker. Instead, we chain the ordered transforms
together within cts-schemas.

We can now also drop the legacy stylesheet symlinks, which were only
used by cts-schemas.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>