src/testing/benchmark.go: add subset implementation of Benchmark

Partially fixes #1808

Allows the following to succeed:

curl "https://golang.org/test/fibo.go?m=text" > fibo.go
tinygo build -o fibo fibo.go
./fibo -bench

tests/tinygotest/benchmark_test.go

@@ -0,0 +1,50 @@
+// Copyright 2011 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 testbench
+
+import (
+	"testing"
+)
+
+var buf = make([]byte, 13579)
+
+func NonASCII(b []byte, i int, offset int) int {
+	for i = offset; i < len(b) + offset; i++ {
+		if b[i % len(b)] >= 0x80 {
+			break
+		}
+	}
+	return i
+}
+
+func BenchmarkFastNonASCII(b *testing.B) {
+	var val int
+	for i := 0; i < b.N; i++ {
+		val += NonASCII(buf, 0, 0)
+	}
+}
+
+func BenchmarkSlowNonASCII(b *testing.B) {
+	var val int
+	for i := 0; i < b.N; i++ {
+		val += NonASCII(buf, 0, 0)
+		val += NonASCII(buf, 0, 1)
+	}
+}
+
+// TestBenchmark simply uses Benchmark twice and makes sure it does not crash.
+func TestBenchmark(t *testing.T) {
+	// FIXME: reduce runtime from the current 3 seconds.
+	rslow := testing.Benchmark(BenchmarkSlowNonASCII)
+	rfast := testing.Benchmark(BenchmarkFastNonASCII)
+	tslow := rslow.NsPerOp()
+	tfast := rfast.NsPerOp()
+
+	// Be exceedingly forgiving; do not fail even if system gets busy.
+	speedup := float64(tslow) / float64(tfast)
+	if speedup < 0.3 {
+		t.Errorf("Expected speedup >= 0.3, got %f", speedup)
+	}
+}