types/dynamicpb: add NewEnumType

Add support for creating dynamic EnumTypes.

Change-Id: Ic9f5b73630734848b29cc436f6c179549a8ea74a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/237219
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/testing/prototest/enum.go b/testing/prototest/enum.go
new file mode 100644
index 0000000..9ee422e
--- /dev/null
+++ b/testing/prototest/enum.go
@@ -0,0 +1,33 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package prototest
+
+import (
+	"testing"
+
+	pref "google.golang.org/protobuf/reflect/protoreflect"
+)
+
+// Enum tests an EnumType implementation.
+type Enum struct{}
+
+func (test Enum) Test(t testing.TB, et pref.EnumType) {
+	ed := et.Descriptor()
+	values := ed.Values()
+	for i := 0; i < values.Len(); i++ {
+		evd := values.Get(i)
+		num := evd.Number()
+		e := et.New(num)
+		if e.Descriptor() != ed {
+			t.Errorf("enumType.New(%v).Descriptor() != enumType.Descriptor(), should match", num)
+		}
+		if e.Type() != et {
+			t.Errorf("enumType.New(%v).Type() != enumType, should match", num)
+		}
+		if got, want := e.Number(), num; got != want {
+			t.Errorf("enumType.New(%v).Number() = %v, want %v", num, got, want)
+		}
+	}
+}