Use SKIPPED_TEST_EXIT_CODE consistently

Use SKIPPED_TEST_EXIT_CODE consistently

Commit fa505bfb0e7623d7cfc94ae15a0246ae71000904 omitted two unit tests
while introducing the SKIP_TEST_EXIT_CODE preprocessor macro.  Fix the
outliers to make use of SKIP_TEST_EXIT_CODE consistent across all unit
tests.  Also make sure lib/dns/tests/dnstap_test returns an exit code
that indicates a skipped test when dnstap is not enabled.