semaphore: avoid loop variable capture by closure

Change-Id: I21c5dc48ffe43aa27c50a42f9c9a01ac1f0c747c
Reviewed-on: https://go-review.googlesource.com/40991
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Glenn Lewis <gmlewis@google.com>

semaphore/semaphore_test.go

@@ -33,6 +33,7 @@ func TestWeighted(t *testing.T) {
 	var wg sync.WaitGroup
 	wg.Add(n)
 	for i := 0; i < n; i++ {
+		i := i
 		go func() {
 			defer wg.Done()
 			HammerWeighted(sem, int64(i), 1000)