runtime: Reduce testing for fastlog2 implementation

The current fastlog2 testing checks all 64M values in the domain of
interest, which is too much for platforms with no native floating point.

Reduce testing under testing.Short() to speed up builds for those platforms.

Related to #12620

Change-Id: Ie5dcd408724ba91c3b3fcf9ba0dddedb34706cd1
Reviewed-on: https://go-review.googlesource.com/15830Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Joel Sing <jsing@google.com>
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/fastlog2_test.go

@@ -15,7 +15,13 @@ func TestFastLog2(t *testing.T) {
 	// implementation over the range of interest for heap sampling.
 	const randomBitCount = 26
 	var e float64
-	for i := 1; i < 1<<randomBitCount; i++ {
+
+	inc := 1
+	if testing.Short() {
+		// Check 1K total values, down from 64M.
+		inc = 1 << 16
+	}
+	for i := 1; i < 1<<randomBitCount; i += inc {
 		l, fl := math.Log2(float64(i)), runtime.Fastlog2(float64(i))
 		d := l - fl
 		e += d * d