tests: time_units: check for overflow in z_tmcvt intermediate

Prior to #41602, due to the ordering of operations (first mul,
then div), an intermediate value would overflow, resulting in
a time non-linearity.

This test ensures that time rolls-over properly.

Signed-off-by: Chris Friedt <cfriedt@meta.com>
(cherry picked from commit 74c9c0e7)