cmd/link/internal/dwtest: fix inconsistent receiver name

cmd/link/internal/dwtest: fix inconsistent receiver name

The DIEs method of Examiner is the only one with a receiver declared 'e'; all the rest have 'ex'. Fix it to be consistent.

Change-Id: I494a53cf3db5575d45831cab3cf612c20962f8fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/554575
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: shuang cui <imcusg@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>