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

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

lib/dns/tests/update_test was failing on macOS on random occasions.  It
turned out this was a linker problem - it preferred isc_stdtime_get()
from libisc instead of the local version in lib/dns/tests/update_test.c.

Fix by including the original .c file in the unit test.  This has two
benefits:

 a) linking order may no longer cause issues as symbols found in the
    same compilation unit are always preferred,

 b) it allows writing tests for static functions in lib/dns/update.c.