runtime: account for tiny allocs, for testing.AllocsPerRun

Fixes #8734.

LGTM=r, bradfitz, dvyukov
R=bradfitz, r, dvyukov
CC=golang-codereviews, iant, khr
https://golang.org/cl/143150043

src/testing/allocs_test.go

@@ -0,0 +1,29 @@
+// 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.
+
+package testing_test
+
+import "testing"
+
+var global interface{}
+
+var allocsPerRunTests = []struct {
+	name   string
+	fn     func()
+	allocs float64
+}{
+	{"alloc *byte", func() { global = new(*byte) }, 1},
+	{"alloc complex128", func() { global = new(complex128) }, 1},
+	{"alloc float64", func() { global = new(float64) }, 1},
+	{"alloc int32", func() { global = new(int32) }, 1},
+	{"alloc byte", func() { global = new(byte) }, 1},
+}
+
+func TestAllocsPerRun(t *testing.T) {
+	for _, tt := range allocsPerRunTests {
+		if allocs := testing.AllocsPerRun(100, tt.fn); allocs != tt.allocs {
+			t.Errorf("AllocsPerRun(100, %s) = %v, want %v", tt.name, allocs, tt.allocs)
+		}
+	}
+}