oauth2/google: simplify the mechanism for overriding gcloud config location

Change-Id: I360fac6b13d11dda221beaa6b46ff386adfc2ec8
Reviewed-on: https://go-review.googlesource.com/4670
Reviewed-by: David Symonds <dsymonds@golang.org>
Run-TryBot: David Symonds <dsymonds@golang.org>

google/sdk_test.go

@@ -7,7 +7,10 @@ package google
 import "testing"
 
 func TestSDKConfig(t *testing.T) {
-	unixHomeDir = "testdata"
+	sdkConfigPath = func() (string, error) {
+		return "testdata/gcloud", nil
+	}
+
 	tests := []struct {
 		account     string
 		accessToken string