runtime: only run TestArenaCollision if the target can exec

Replace the test for nacl with testenv.MustHaveExec to also skip
test on iOS.

Change-Id: I6822714f6d71533d1b18bbb7894f6ad339d8aea1
Reviewed-on: https://go-review.googlesource.com/94755
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/malloc_test.go

@@ -163,9 +163,8 @@ type acLink struct {
 var arenaCollisionSink []*acLink
 
 func TestArenaCollision(t *testing.T) {
-	if GOOS == "nacl" {
-		t.Skip("nacl can't self-exec a test")
-	}
+	testenv.MustHaveExec(t)
+
 	// Test that mheap.sysAlloc handles collisions with other
 	// memory mappings.
 	if os.Getenv("TEST_ARENA_COLLISION") != "1" {