tests: Skip trapezoids for unsupported XRender formats
xserver commit 436fd7e8b4966c305ea9c43f3c14c2ca04c35539 removed support
for PictStandardA4. x11perf assumes this format always exists, so trying
to run the -aa4trap* tests causes a segmentation fault when
XRenderFindStandardFormat (xp->d, PictStandardA4)
returns NULL.
Since the server change was intentional, skip the tests if XRenderFindStandardFormat() returns NULL.
See also: xorg/xserver@436fd7e8
--
I considered printing a message in this case but most tests seem to be silently skipped if anything goes wrong, with the exception of the do_text.c tests.