libvirt-tests: Add test case for nodesuspend

Test command: `virsh nodesuspend <target> <duration>`

This command will make host suspend or hibernate, running tests on testing
grid may cause unexpected behavior.

This tests only work when test runner setup a remote host (physical or
virtual) with testing version of libvirt daemon running. After that change
the `remote_xxx` parameters in configuration file to corresponding value.

Signed-off-by: Hao Liu <hliu@redhat.com>
I saw there are 2 `whlie` cycles for `ping` command, how about using `virttests.utils_test.ping()`, its parameters has covered yours, `dest, count, timeout`