iotests/308: Test +w on read-only FUSE exports

Test that +w on read-only FUSE exports returns an EROFS error.  u+x on
the other hand should work.  (There is no special reason to choose u+x
here, it simply is like +w another flag that is not set by default.)

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210625142317.271673-6-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>