tests: fail make check if nproc is too low

tests: fail make check if nproc is too low

When running tests in parallel with make -jX, the ulimit -u (number of
processor / thread per user) needs to be at least X * 1024. If not it
will fail in mysterious ways. Since there is no convenient way to figure
out the value of X ( see
http://blog.jgc.org/2015/03/gnu-make-insanity-finding-value-of-j.html
for a non trivial an entertaining solution) add a very conservative
check that assumes the user will run make -jX where X is nproc / 2.

It will be annoying for users who want to run make check, not use -j,
and have a low ulimit -u. But the error suggest a way to override this
with

   make CHECK_ULIMIT=false check

This is a minor irritation compared to the puzzling behavior of make
check when ulimit is exceeded.

http://tracker.ceph.com/issues/11532 Fixes: #11532

Signed-off-by: Loic Dachary <ldachary@redhat.com>