[dev.typeparams] test: add scaffolding to run.go to check compiler with -G flag

Added a new flag -G to run. Setting -G (as in: go run run.go -G)
will run tests marked with "errorcheck" (and no other flags) also
with the compiler using the new typechecker.

Many tests don't pass yet (due to discrepancies in error messages).
The top-level tests in the test directory which don't pass yet have
been explicitly excluded, permitting to see the current status.
Future CLs will bring error messages in sync and eventually all
tests should pass.

Change-Id: I7caf5eff413e173f68d092af4bbe458434718d74
Reviewed-on: https://go-review.googlesource.com/c/go/+/274313


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>