test: add some tests for mismatches between call results and uses

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://golang.org/cl/111360045

test/fixedbugs/bug487.go

+// errorcheck
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The gccgo compiler did not reliably report mismatches between the
+// number of function results and the number of expected results.
+
+package p
+
+func G() (int, int, int) {
+	return 0, 0, 0
+}
+
+func F() {
+	a, b := G()	// ERROR "mismatch"
+	a, b = G()	// ERROR "mismatch"
+	_, _ = a, b
+}
+
+func H() (int, int) {
+	return G()	// ERROR "too many|mismatch"
+}