providers/heroku: fix drain test

cc/ @bmarini

Basically, the attr check we have in our tests is looking at resources
that we specify. It's basically check that "if I send this, I get this"
from the external resource.

In this case, we don't know what the token will be, so we can't
do it that way. I replaced it with a "make sure this is more than an
empty string" assertion.

builtin/providers/heroku/resource_heroku_drain_test.go

@@ -26,8 +26,6 @@ func TestAccHerokuDrain_Basic(t *testing.T) {
 						"heroku_drain.foobar", "url", "syslog://terraform.example.com:1234"),
 					resource.TestCheckResourceAttr(
 						"heroku_drain.foobar", "app", "terraform-test-app"),
-					resource.TestCheckResourceAttr(
-						"heroku_drain.foobar", "token", "foo-bar-baz-qux"),
 				),
 			},
 		},
@@ -59,6 +57,10 @@ func testAccCheckHerokuDrainAttributes(Drain *heroku.LogDrain) resource.TestChec
 			return fmt.Errorf("Bad URL: %s", Drain.URL)
 		}
 
+		if Drain.Token == "" {
+			return fmt.Errorf("No token: %#v", Drain)
+		}
+
 		return nil
 	}
 }