Test time funcs

Right now there's one test for each of the C++ time funcs. They are arguably undertested, and the C time funcs are not tested at all.

How should the C versions even be tested, if they call `time()` and `timespec_get()`?

Start by locating the existing one-test-per-C++-time-function coverage and the untested C time functions in the test suite. Determine how calls to time() and timespec_get() can be tested reliably; done means both the C++ and C variants have meaningful coverage without depending on unstable wall-clock results.

Written by the indexing model from the issue text.