test/try_open: Drop openslide_can_open() test
test/try_open: Drop openslide_can_open() test

openslide_can_open() now uses essentially the same code paths as
openslide_open(), so it's not as important that we test it exhaustively.
It's deprecated, so we want to silence the compiler warning.  And
removing it will speed up testsuite runs.

openslide/openslide,openslide/openslide,openslide/openslide,openslide/openslide