tests: lib: timeutil: add timespec util testsuite

Add a timespec util testsuite. This should have reasonably high enough
coverage to be useful.

I would have preferred to add this as an architecture-independent
unit test (for the unit_testing platform) under tests/unit/timeutil but
there is an inconsistency about the size of time_t on the unit_testing
and native_sim/native platforms. On every other platform supported by
Zephyr, time_t is 64-bits. However, on those platforms, time_t is only
32-bits.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>