The test added in CL 420394 only tested that the type assertions
compiled at all. This CL changes it into a run test to make sure the
type assertions compile and also run correctly.
Updates #54135.
Change-Id: Id17469faad1bb55ff79b0bb4163ef50179330033
Reviewed-on: https://go-review.googlesource.com/c/go/+/420421
Run-TryBot: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Keith Randall <khr@google.com>