[dev.typeparams] cmd/compile/internal/syntax: add ShortString tests

[dev.typeparams] cmd/compile/internal/syntax: add ShortString tests

This CL moves the exprstring_test.go from the types2
package into the syntax package (which contains the
actual ShortString function). The code is mostly un-
changed but for the updated TestShortString function.

Change-Id: Ib39e3181e643fc0ac96ddf144a3114893a50c2fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/282554
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>