base: add a test for NTP

By using ntpstat tool, we can check the return code.
exit status 0 - Clock is synchronised.
exit status 1 - Clock is not synchronised.
exit status 2 - If clock state is indeterminant, for example if ntpd is not contactable.

So this patch aims to test if ntpstat returns '0'.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>