tests/9p: add 'use-after-unlink' test

tests/9p: add 'use-after-unlink' test

After removing a file from the file system, we should still be able to
work with the file if we already had it open before removal.

As a first step we verify that it is possible to write to an unlinked
file, as this is what already works. This test is extended later on
after having fixed other use cases after unlink that are not working
yet.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <3d6449d4df25bcdd3e807eff169f46f1385e5257.1732465720.git.qemu_oss@crudebyte.com>