runtime: enable TestEmptySlice

It says to disable until #7564 is fixed. It was fixed in April 2014.

Change-Id: I9bebfe96802bafdd2d1a0a47591df346d91b000c
Reviewed-on: https://go-review.googlesource.com/12858
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/mfinal_test.go

@@ -171,9 +171,6 @@ func adjChunks() (*objtype, *objtype) {
 
 // Make sure an empty slice on the stack doesn't pin the next object in memory.
 func TestEmptySlice(t *testing.T) {
-	if true { // disable until bug 7564 is fixed.
-		return
-	}
 	x, y := adjChunks()
 
 	// the pointer inside xs points to y.