[dev.unified] test: add regress test for generic select statements

[dev.unified] test: add regress test for generic select statements

The Go 1.18 frontend ICEs on select case clauses that involve an
implicit conversion.

Change-Id: I1c0865bf97d8b0a8fbddb0da43333e909df0d38a
Reviewed-on: https://go-review.googlesource.com/c/go/+/414878
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: David Chase <drchase@google.com>