cmd/go: add t.Helper calls to test helpers

Now that we have t.Helper, might as well use it to make the
reported failure lines more helpful.

Change-Id: I2a0c64e9ca7bdc0eaf2b62f9f855c41467767084
Reviewed-on: https://go-review.googlesource.com/56274
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>