cmd/go: test whether alldocs.go is up to date

A common error is to update the help text for a command in cmd/go, but
fail to update alldocs.go, which actually prints the help text for the
most common commands.

Add a test that the long-form documentation help text matches the
contents of alldocs.go, which will fail the build if we fail to keep
the documentation in sync. We can get fancier with the test output if
this is not sufficient.

Fixes golang/go#26735.

Change-Id: I2509765315eeb0f362633d812343d1324a01b73b
Reviewed-on: https://go-review.googlesource.com/127920


Run-TryBot: Kevin Burke <kev@inburke.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>