On baremetal platforms, use simpler test matcher. Fixes #2666.

Uses same matcher in testdata (because now we can't replace it in testdata).

src/testing/is_baremetal.go

@@ -0,0 +1,10 @@
+// Copyright 2016 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.
+//
+//go:build baremetal
+// +build baremetal
+
+package testing
+
+const isBaremetal = true