[dev.typeparams] go/types: add missing test from dev.go2go

errors_test.go was missed during merging. Add it.

Change-Id: I321f08ae16ca02586875e1c7776f5d78f8690b4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/289549
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/errors_test.go

@@ -0,0 +1,25 @@
+// Copyright 2020 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.
+
+package types
+
+import "testing"
+
+func TestStripAnnotations(t *testing.T) {
+	for _, test := range []struct {
+		in, want string
+	}{
+		{"", ""},
+		{"   ", "   "},
+		{"foo", "foo"},
+		{"foo₀", "foo"},
+		{"foo(T₀)", "foo(T)"},
+		{"#foo(T₀)", "foo(T)"},
+	} {
+		got := stripAnnotations(test.in)
+		if got != test.want {
+			t.Errorf("%q: got %q; want %q", test.in, got, test.want)
+		}
+	}
+}