The code was already behaving like that but there was XXX to do it. Add
test to verify it is actually done.

Opened WatchLink handle is released after RELEASE because
read in WatchLink.serve, after RELEASE, returns EOF and then the code
inside WCFS does all necessary WatchLink-related cleanup:

https://lab.nexedi.com/nexedi/wendelin.core/-/blob/wendelin.core-2.0.alpha3-26-g79e6f7b9/wcfs/wcfs.go#L1828-1872

/reviewed-by @levin.zimmermann
/reviewed-on nexedi/wendelin.core!18