In unittests.cfg one can add a line like the following:
check = /proc/sys/kernel/nmi_watchdog=0 /proc/sys/kernel/ostype=Linux

run_tests.sh will now check for those values (if defined) and only run
the test if all conditions are true.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>