At the moment, the cover tool does not check that the argument of -var
is a valid identifier. Hence, it could generate a file that fails to
compile afterwards.

Updates #25280

Change-Id: I6eb1872736377680900a18a4a28ba002ab5ea8ca
Reviewed-on: https://go-review.googlesource.com/c/120316
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>