Fix fmt verbs for strings in roundtrip test errors.

Some of the calls to t.Errorf from apitesting/roundtrip use the %#v verb (Go-syntax representation)
for string arguments that are have already been formatted for legibility by something like like
cmp.Diff. Quoting and escaping newlines in these strings makes them harder to read, like this:

> ...
> "(*storage.CSIDriver)({\n TypeMeta: (v1.TypeMeta) {\n Kind: (string) \"\",\n APIVersion: (string)
> \"\"\n },\n ObjectMeta: (v1.ObjectMeta) {\n Name: (string) (len=15) \"犱âM邽[ǎ*ʄ\",\n
> ...

Kubernetes-commit: 6c18d0ec2e01c69c79a005e8ce4194567b945cc2

Author: benluddy

pkg/api/apitesting/roundtrip/roundtrip.go

@@ -350,14 +350,14 @@ func roundTrip(t *testing.T, scheme *runtime.Scheme, codec runtime.Codec, object
 	// decode (deserialize) the encoded data back into an object
 	obj2, err := runtime.Decode(codec, data)
 	if err != nil {
-		t.Errorf("%v: %v\nCodec: %#v\nData: %s\nSource: %#v", name, err, codec, dataAsString(data), dump.Pretty(object))
+		t.Errorf("%v: %v\nCodec: %#v\nData: %s\nSource: %s", name, err, codec, dataAsString(data), dump.Pretty(object))
 		panic("failed")
 	}
 
 	// ensure that the object produced from decoding the encoded data is equal
 	// to the original object
 	if !apiequality.Semantic.DeepEqual(original, obj2) {
-		t.Errorf("%v: diff: %v\nCodec: %#v\nSource:\n\n%#v\n\nEncoded:\n\n%s\n\nFinal:\n\n%#v", name, cmp.Diff(original, obj2), codec, dump.Pretty(original), dataAsString(data), dump.Pretty(obj2))
+		t.Errorf("%v: diff: %v\nCodec: %#v\nSource:\n\n%s\n\nEncoded:\n\n%s\n\nFinal:\n\n%s", name, cmp.Diff(original, obj2), codec, dump.Pretty(original), dataAsString(data), dump.Pretty(obj2))
 		return
 	}