encoding/textpb: reduce dependency on proto v1 package

Substitute protoV1.Marshal calls to use proto v2.

A few tests still need to use v1 due to missing required field checks in
v2.

Change-Id: I97ee208b81afc85c81142e70c1d7bdc47db76ce4
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/167774
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/encoding/textpb/encode_test.go b/encoding/textpb/encode_test.go
index 3430fc5..6a6cd7a 100644
--- a/encoding/textpb/encode_test.go
+++ b/encoding/textpb/encode_test.go
@@ -1029,8 +1029,7 @@
 					OptString: scalar.String("inception"),
 				},
 			}
-			// TODO: Switch to V2 marshal when ready.
-			b, err := protoV1.Marshal(m)
+			b, err := proto.MarshalOptions{Deterministic: true}.Marshal(m)
 			if err != nil {
 				t.Fatalf("error in binary marshaling message for Any.value: %v", err)
 			}
@@ -1054,8 +1053,7 @@
 					OptString: scalar.String("inception"),
 				},
 			}
-			// TODO: Switch to V2 marshal when ready.
-			b, err := protoV1.Marshal(m)
+			b, err := proto.MarshalOptions{Deterministic: true}.Marshal(m)
 			if err != nil {
 				t.Fatalf("error in binary marshaling message for Any.value: %v", err)
 			}