presubmit: Check errorf as well (#1845)

Uppercase all these test errors as well. And extend the presubmit to
check for these in the future. Also do a slightly smarter grep to only
get t.<something>. as (because dump regexp) this also grep over non test
files.

plugin/file/ent_test.go

@@ -45,7 +45,7 @@ func TestLookupEnt(t *testing.T) {
 		rec := dnstest.NewRecorder(&test.ResponseWriter{})
 		_, err := fm.ServeDNS(ctx, rec, m)
 		if err != nil {
-			t.Errorf("expected no error, got %v\n", err)
+			t.Errorf("Expected no error, got %v\n", err)
 			return
 		}