[dev.typeparams] cmd/compile/internal/types: review of typestring_test.go

The changes between (equivalent, and reviewed) go/types/typestring_test.go
and typestring_test.go can be seen by comparing patchset 1 and 2. The actual
change is just removing the "// UNREVIEWED" marker.

Change-Id: I66150c0ab738763d2d8b94483ef8314cbe28a374
Reviewed-on: https://go-review.googlesource.com/c/go/+/293473


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>