testutil: add ShouldNotAllocate helper

internal/testutil/allocs.go

@@ -0,0 +1,16 @@
+package testutil
+
+import (
+	"testing"
+)
+
+// ShouldNotAllocate fails if f allocates.
+func ShouldNotAllocate(t *testing.T, f func()) {
+	if Race {
+		t.Skip("skip while running with -race")
+		return
+	}
+	if a := testing.AllocsPerRun(10, f); a > 0 {
+		t.Errorf("allocations detected: %f", a)
+	}
+}