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 <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

Author: stamblerre

internal/testenv/testenv.go

@@ -75,7 +75,11 @@ func NeedsGoPackages(t Testing) {
 	}
 
 	tool := os.Getenv("GOPACKAGESDRIVER")
-	if tool == "" {
+	switch tool {
+	case "off":
+		// "off" forces go/packages to use the go command.
+		tool = "go"
+	case "":
 		if _, err := exec.LookPath("gopackagesdriver"); err == nil {
 			tool = "gopackagesdriver"
 		} else {