Add test case for duplicate symbols from InterfaceBlocks.

This works fine in practice, but we didn't have any tests that
exercised this case.

Change-Id: I691b81175f43cd346c29a44d16b5712ec6de270f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/592377
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
3 files changed