getloadavg test: skip the test on GNU/Linux without /proc mounted

getloadavg test: skip the test on GNU/Linux without /proc mounted

GNU libc implements getloadavg(3) on Linux by parsing /proc/loadavg
file.  When /proc is not mounted, it always fails with ENOENT.
* tests/test-getloadavg.c (main): Treat ENOENT return code from
getloadavg(3) the same way as ENOSYS and ENOTSUP.