Use global rand.Rand instance in mathrand.

common/data/rand/mathrand/mathrand_test.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package mathrand
 
 import (
         "math"
         "math/rand"
         "testing"
 
         . "github.com/smartystreets/goconvey/convey"
         "golang.org/x/net/context"
 )
 
 func Test(t *testing.T) {
         t.Parallel()
 
         Convey("test mathrand", t, func() {
                 c := context.Background()
 
                 Convey("unset", func() {
                         // Just ensure doesn't crash.
                         So(Get(c).Int()+1 > 0, ShouldBeTrue)
+                        So(WithGoRand(c, func(r *rand.Rand) error {
+                                So(r.Int(), ShouldBeGreaterThanOrEqualTo, 0)
+                                return nil
+                        }), ShouldBeNil)
                 })
 
                 Convey("set persistance", func() {
                         c = Set(c, rand.New(rand.NewSource(12345)))
                         r := rand.New(rand.NewSource(12345))
                         So(Get(c).Int(), ShouldEqual, r.Int())
                         So(Get(c).Int(), ShouldEqual, r.Int())
                 })
 
                 Convey("nil set", func() {
@@ skipping 73 matching lines @@
                 })
                 return nil
         })
 }
 
 // BenchmarkStdlibDefaultSource-8               50000000                32.0 ns/op
 // BenchmarkOurDefaultSourceViaCtx-8              200000               10893 ns/op
 // BenchmarkOurDefaultSourceViaFunc-8           30000000                38.7 ns/op
 // BenchmarkOurInitializedSourceViaCtx-8        50000000                30.2 ns/op
 // BenchmarkOurInitializedSourceViaFunc-8       50000000                29.1 ns/op