internal/testenv: handle GOPACKAGESDRIVER=off in NeedsGopackage
internal/testenv: handle GOPACKAGESDRIVER=off in NeedsGopackage

This was causing tests to report that they required a binary
called "off".

Change-Id: I592109029c86af20d48d2ee9989d4a25fdb11b3d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/194978
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools