When testing, check if '/proc/mounts' exists. (by Bojan Petrovic)

When testing, check if '/proc/mounts' exists. (by Bojan Petrovic)

On FreeBSD, even if procfs is mounted under '/proc', there is no
'/proc/mounts' file.  Instead on relying on the existence of '/proc'
directory, we check if the '/proc/mounts' file exists. Otherwise, tests
cannot even start on FreeBSD.