testing/prototest: refactor prototest API

For consistency with other options types in the protobuf module, make
the test function a method of the options.

Drop the ExtensionTypes option and just look up the extension types to
test with in the provided resolver.

Change-Id: I7918bd10b7c003e4af56d27521d30218653d5b4d
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/219142
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/testing/prototest/prototest_test.go b/testing/prototest/prototest_test.go
index 44fa0bc..4307d44 100644
--- a/testing/prototest/prototest_test.go
+++ b/testing/prototest/prototest_test.go
@@ -38,7 +38,7 @@
 
 	for _, m := range ms {
 		t.Run(fmt.Sprintf("%T", m), func(t *testing.T) {
-			prototest.TestMessage(t, m, prototest.MessageOptions{})
+			prototest.Message{}.Test(t, m.ProtoReflect().Type())
 		})
 	}
 }