iotests/287: Clean up subshell test image

287 creates an image in a subshell (thanks to the pipe) to see whether
that is possible with compression_type=zstd.  If _make_test_img were to
modify any global state, this global state would then be lost before we
could cleanup the image.

When using FUSE as the test protocol, this global state is important, so
clean up the image before the state is lost.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201027190600.192171-16-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>