cmd/compile: set unsayable "names" for regabi testing triggers

This disables the "testing names" for method names and
trailing input types passed to closure/interface/other calls.
The logic using the names remains, so that editing the change
to enable local testing is not too hard.

Also fixes broken build tag in reflect/abi_test.go

Updates #44816.

Change-Id: I3d222d2473c98d04ab6f1122ede9fea70c994af1
Reviewed-on: https://go-review.googlesource.com/c/go/+/300150


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>