bytes: Benchmark Buffer's compactions

This benchmark verifies that CL #8173043 reduces time spent
sliding the Buffer's contents.

Results without and with CL #8173043 applied:
benchmark                        old ns/op    new ns/op    delta
BenchmarkBufferFullSmallReads       755336       175054  -76.82%

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/8174043

src/pkg/bytes/buffer_test.go

@@ -509,3 +509,19 @@ func BenchmarkBufferNotEmptyWriteRead(b *testing.B) {
 		}
 	}
 }
+
+// Check that we don't compact too often. From Issue 5154.
+func BenchmarkBufferFullSmallReads(b *testing.B) {
+	buf := make([]byte, 1024)
+	for i := 0; i < b.N; i++ {
+		var b Buffer
+		b.Write(buf)
+		for b.Len()+20 < b.Cap() {
+			b.Write(buf[:10])
+		}
+		for i := 0; i < 5<<10; i++ {
+			b.Read(buf[:1])
+			b.Write(buf[:1])
+		}
+	}
+}