tests/9pfs: check fid being unaffected in fs_walk_2nd_nonexistent

tests/9pfs: check fid being unaffected in fs_walk_2nd_nonexistent

Extend previously added test case by checking that fid was unaffected
by 'Twalk' request (i.e. when 2nd path component of request being
invalid). Do that by subsequently sending a 'Tgetattr' request with
the fid previously used for 'Twalk'; that 'Tgetattr' request should
return an 'Rlerror' response by 9p server with error code ENOENT as
that fid is basically invalid.

And as we are at it, also check that the QID returned by 'Twalk' is
not identical to the root node's QID.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <6f0813cafdbf683cdac8b1492dd4ef8699c5b1d9.1647339025.git.qemu_oss@crudebyte.com>