selftests/nolibc: condition some tests on /proc existence

If /proc is not available (program run inside a chroot or without
sufficient permissions), it's better to disable the associated tests.
Some will be preserved like the ones which check for a failure to
create some entries there since they're still supposed to fail.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>