tests/qemu-iotests/tests/mirror-sparse: skip if O_DIRECT is not supported

tests/qemu-iotests/tests/mirror-sparse: skip if O_DIRECT is not supported

This test uses cache.direct=true, but does not check if O_DIRECT
is supported by the underlying filesystem, and fails, for example,
on a tmpfs (which is rather common on various auto-builders, in CI,
etc).

Fix this by using `_supported_cache_modes none directsync`.

Fixes: c0ddcb2cbc146e "tests: Add iotest mirror-sparse for recent patches"
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20250805181731.282677-1-mjt@tls.msk.ru>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>