testing: shorten some tests.

These are the top runners.  More to come.
Also print two digits of timing info under -test.v.

R=rsc
CC=golang-dev
https://golang.org/cl/4317044

src/pkg/bytes/bytes_test.go

@@ -201,7 +201,10 @@ func TestIndexByte(t *testing.T) {
 
 // test a larger buffer with different sizes and alignments
 func TestIndexByteBig(t *testing.T) {
-	const n = 1024
+	var n = 1024
+	if testing.Short() {
+		n = 128
+	}
 	b := make([]byte, n)
 	for i := 0; i < n; i++ {
 		// different start alignments