tests: Fix failing after year 2038

[andy5995]

For #439

[andy5995]

@bmwiedemann Are you able to test this patch easily before I merge it?

I've submitted the patch to the OpenSuse rmw package here https://build.opensuse.org/request/show/1161718

[bmwiedemann]

It passes the test now, but I still see room for improvement. E.g.
long prev_time = atol(time_prev);
will use 32-bit integers on 32-bit platforms - armv7 (Raspberry Pi) is the most common one.
Better use long long and atoll instead.