tests: move /proc/ checks from scripts to executables

tests: move /proc/ checks from scripts to executables

Move most of /proc/self/task/ and /proc/self/fd/ checks from scripts
to xecutables.

* tests/attach-f-p.test: Move /proc/self/task/ check ...
* tests/attach-f-p-cmd.c (main): ... here.
* tests/fstat.test: Move /proc/self/fd/ check ...
* tests/xstatx.c (main): ... here.
* tests/net-y-unix.test: Move /proc/self/fd/ check ...
* tests/net-y-unix.c (main): ... here.
* tests/net-yy-inet.test: Move /proc/self/fd/ check ...
* tests/net-yy-inet.c (main): ... here.
* tests/net-yy-netlink.test: Move /proc/self/fd/ check ...
* tests/net-yy-netlink.c (main): ... here.
* tests/net-yy-unix.test: Move /proc/self/fd/ check ...
* tests/net-yy-unix.c (main): ... here.