tests: Make test for Linux /proc/self/fd more straightforward.

tests: Make test for Linux /proc/self/fd more straightforward.

Presumably we can test for this Linux feature just by seeing whether the
directory is there.

Another goal is to shorten the code because I intend to make another copy
of it in an upcoming commit, to add a similar test for Python.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>