iotests/303: Check for zstd support

iotests/303: Check for zstd support

303 runs two test cases, one of which requires zstd support.
Unfortunately, given that this is not a unittest-style test, we cannot
easily skip that single case, and instead can only skip the whole test.

(Alternatively, we could split this test into a zlib and a zstd part,
but that seems excessive, given that this test is not in auto and thus
likely only run by developers who have zstd support compiled in.)

Fixes: 677e0bae686e7c670a71d1f ("iotest 303: explicit compression type")
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <v.sementsov-og@mail.ru>
Message-Id: <20220323105522.53660-4-hreitz@redhat.com>