test: add new test case for 51219 that triggers the types2 issue

The existing test for 51219 didn't actually trigger the types2 issue - I
hadn't been able to minimize the test case yet properly. This new test
case issue51219b.go now does trigger the types2 issue (it's only
slightly different).

Updates #51219

Change-Id: Iaba8144b4702ff4fefec86c899b8acef127b10dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/387814


Trust: Dan Scales <danscales@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>