test: add extra typeswitch tests that cause duplicate cases

Augmented some of the typeswitch*.go tests so that some instantiations
have duplicate cases, in order to ensure we're testing that.

Spacing changes in the tests are due to gofmt.

Change-Id: I5d3678813505c520c544281d4ac8a62ce7e236ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/370155


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>