Use t.Fatalf and t.Errorf in onnxruntime_test.go

- I used t.Logf(), t.FailNow() in many places when a single function would
   have sufficed.  Apparently I need to read the Go docs a bit better.

CONTRIBUTING.md

@@ -53,6 +53,9 @@ Tests
  - In short, after _every_ change, `go test -v -bench=.` must pass on _every_
    supported platform.
 
+ - Every test failure should be accompanied by a message containing the reason,
+   either using `t.Logf()`, `t.Errorf()`, or `t.Fatalf()`.
+
 
 Adding New Files
 ----------------