math/rand: shorten Float32 test for GOARM=5

Fixes #10749

Change-Id: I9d5f6f179fd117b0c358d7c8042daf5985b645c0
Reviewed-on: https://go-review.googlesource.com/10022Reviewed-by: Dave Cheney <dave@cheney.net>

src/math/rand/rand_test.go

@@ -8,6 +8,8 @@ import (
 	"errors"
 	"fmt"
 	"math"
+	"os"
+	"runtime"
 	"testing"
 )
 
@@ -322,10 +324,17 @@ func TestExpTables(t *testing.T) {
 	}
 }
 
-// For issue 6721, the problem came after 7533753 calls, so check 10e6.
 func TestFloat32(t *testing.T) {
+	// For issue 6721, the problem came after 7533753 calls, so check 10e6.
+	num := int(10e6)
+	// But ARM5 floating point emulation is slow (Issue 10749), so
+	// do less for that builder:
+	if testing.Short() && runtime.GOARCH == "arm" && os.Getenv("GOARM") == "5" {
+		num /= 100 // 1.72 seconds instead of 172 seconds
+	}
+
 	r := New(NewSource(1))
-	for ct := 0; ct < 10e6; ct++ {
+	for ct := 0; ct < num; ct++ {
 		f := r.Float32()
 		if f >= 1 {
 			t.Fatal("Float32() should be in range [0,1). ct:", ct, "f:", f)