cmd/compile/internal/types2: add a test for Context deduplication of hash collisions
This CL is a clean port of CL 363517 from go/types to types2,
with the exception that types_test.go was not removed because
it's still needed to set a types2-specific test flag.
Change-Id: I12177866537c0f95f3fa36fa0f4aa02016609ca9
Reviewed-on: https://go-review.googlesource.com/c/go/+/364494
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>