cmd/go: add cmpenv command to testing script language

This will be used by later commits in this sequence.

Updates #28221

Change-Id: I2b22b9f88a0183636cde9509606f03f079eb33f1
Reviewed-on: https://go-review.googlesource.com/c/147277


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>