Also add HasCGO() to internal/testenv for tests.

Updates #21706

Change-Id: I938188047024052bdb42b3ac1a77708f3c2a6dbb
Reviewed-on: https://go-review.googlesource.com/62591
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>