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.

lib/dns/tests/update_test.c

@@ -32,6 +32,13 @@
 
 #include "dnstest.h"
 
+/*
+ * Fix the linking order problem for overridden isc_stdtime_get() by making
+ * everything local.  This also allows static functions from update.c to be
+ * tested.
+ */
+#include "../update.c"
+
 static int
 _setup(void **state) {
 	isc_result_t result;