testing: implement some benchmark stubs

This allows the following packages to pass tests:

  * crypto/des
  * encoding/hex

I have not included crypto/rc4 as it doesn't pass tests on Go 1.11 (but
it works on later versions).

src/testing/benchmark.go

@@ -20,3 +20,15 @@ type InternalBenchmark struct {
 	Name string
 	F    func(b *B)
 }
+
+func (b *B) SetBytes(n int64) {
+	panic("testing: unimplemented: B.SetBytes")
+}
+
+func (b *B) ResetTimer() {
+	panic("testing: unimplemented: B.ResetTimer")
+}
+
+func (b *B) Run(name string, f func(b *B)) bool {
+	panic("testing: unimplemented: B.Run")
+}