text/template: check ignored error in a test

Found with staticcheck. Not terribly important since the test would
likely fail anyway, but at least it will fail with a better explanation
now.

Change-Id: Ic3f9a94a2152404b7873cc8cd47b6db79d78c2e6
Reviewed-on: https://go-review.googlesource.com/62990
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/text/template/multi_test.go

@@ -247,6 +247,9 @@ func TestAddParseTree(t *testing.T) {
 		t.Fatal(err)
 	}
 	added, err := root.AddParseTree("c", tree["c"])
+	if err != nil {
+		t.Fatal(err)
+	}
 	// Execute.
 	var b bytes.Buffer
 	err = added.ExecuteTemplate(&b, "a", 0)