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>
(cherry picked from commit 462db8fb1d405391b83a0d3099fdb9bfb85c2d92)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(Mjt: context fix, pick it to stable so the next patch in this place applies)