Put a proto package on the test file. (#1813)

This one got overlooked in the past, and the usage isn't about the lack
of a proto package in naming, so add one so it is more inline with what
is expected from a .proto file style pov.

Author: thomasvl

Protos/Tests/SwiftProtobufTests/unittest_swift_groups.proto

@@ -30,13 +30,15 @@
 
 syntax = "proto2";
 
+package swift_proto_testing;
+
 // Same field number appears inside and outside of the group.
-message SwiftTestGroupExtensions {
+message GroupExtensions {
   optional int32 a = 1;
   extensions 2 to 10;
 }
 
-extend SwiftTestGroupExtensions {
+extend GroupExtensions {
   optional group ExtensionGroup = 2 {
     optional int32 a = 1;
   }
@@ -45,11 +47,11 @@ extend SwiftTestGroupExtensions {
   }
 }
 
-message SwiftTestGroupUnextended {
+message GroupUnextended {
   optional int32 a = 1;
 }
 
-message SwiftTestNestingGroupsMessage {
+message NestingGroupsMessage {
   optional int32 outer_a = 1;
   optional group SubGroup1 = 2 {
     optional int32 sub1_a = 1;