[dev.typeparams] go/parser: accept "~" and "|" interface elements

This is a port of CL 307371 to go/parser, adding support for the new
embedded type expressions. As in that CL, type lists continue to be
accepted.

This CL also revealed a pre-existing bug related to embedded instances:
the parser was failing to parse embedded instances with multiple type
arguments, due to not consuming the initial ','. This is fixed, and
along the way TestErrors is modified to use subtests.

Several missing tests cases were added to exprstring_test.go. These must
have been missed in an earlier CL.

Change-Id: I452769536998cddb1618bebdba675fc09d48a12f
Reviewed-on: https://go-review.googlesource.com/c/go/+/325690


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>