encoding/prototext: use testmessages_test.go approach, too

With this change, we establish a similar mechanism in encoding/prototext as
already exists in the proto package: a testmessages_test.go which serves as an
extension point where one can inject more variants into the corpus of test
messages.

Change-Id: I9f83735a44e5fd2e4736f888aaca428e4cc8ed42
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/623116
Reviewed-by: Chressie Himpel <chressie@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

encoding/protojson/encode_test.go

@@ -111,8 +111,8 @@ func TestMarshal(t *testing.T) {
   "optUint64": "0",
   "optFloat": 0,
   "optDouble": 0,
-  "optString": "",
   "optBytes": "",
+  "optString": "",
   "optEnum": "ZERO",
   "optMessage": {}
 }`,