runtime/debug: make TestFreeOSMemory repeatable

Fixes #4835.

R=golang-dev, fullung
CC=golang-dev
https://golang.org/cl/7319050

src/pkg/runtime/debug/garbage_test.go

@@ -71,11 +71,12 @@ func TestReadGCStats(t *testing.T) {
 	}
 }
 
-var big = make([]byte, 1<<20)
+var big []byte
 
 func TestFreeOSMemory(t *testing.T) {
 	var ms1, ms2 runtime.MemStats
 
+	big = make([]byte, 1<<20)
 	big = nil
 	runtime.GC()
 	runtime.ReadMemStats(&ms1)