Fix lib/dns/tests/update_test spurious errors on macOS

The update_test was failing on macOS on random ocasions. It turned out this was a linker problem that prefered isc_stdtime_get() from libisc instead of the local copy.

The change adapts an approach that includes the original .c file into the unit tests. This has two benefits:

a) the symbol ordering is absolutely clear (use symbols to the compilation unit) b) it allows writing new tests for the static functions in the lib/dns/update.c

Closes #161 (closed)