libdns: add missing checks for return values in dnstap unit test

libdns: add missing checks for return values in dnstap unit test

Related scan-build report:

dnstap_test.c:169:2: warning: Value stored to 'result' is never read
        result = dns_test_makeview("test", &view);
        ^        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
dnstap_test.c:193:2: warning: Value stored to 'result' is never read
        result = dns_compress_init(&cctx, -1, dt_mctx);
        ^        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 warnings generated.

(cherry picked from commit e9acad638eb21e0ef0bd8558a196ca24c3099292)