tests: skip aes test if elements not built

tests: skip aes test if elements not built

In ext/aes/meson.build, the aes_dep will return
not-found if -Daes=disabled, regardless of whether
openssl is found or not.  Thus, we don't need a
separate check for the option.  This will also
ensure that aes_dep is always defined and we can
use it in the tests/check/meson.build unit.

Fixes #1660

v2: handle -Daes=disabled, too.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2522>