runtime: re-enable TestArenaCollision on darwin in race mode

Go 1.14 will drop support for macOS 10.10, see #23011

This reverts CL 155097

Updates #26475
Updates #29340

Change-Id: I64d0275141407313b73068436ee81d13eacc4c76
Reviewed-on: https://go-review.googlesource.com/c/go/+/214058
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/malloc_test.go

@@ -204,14 +204,6 @@ type acLink struct {
 var arenaCollisionSink []*acLink
 
 func TestArenaCollision(t *testing.T) {
-	if GOOS == "darwin" && race.Enabled {
-		// Skip this test on Darwin in race mode because Darwin 10.10 has
-		// issues following arena hints and runs out of them in race mode, so
-		// MAP_FIXED is used to ensure we keep the heap in the memory region the
-		// race detector expects.
-		// TODO(mknyszek): Delete this when Darwin 10.10 is no longer supported.
-		t.Skip("disabled on Darwin with race mode since MAP_FIXED is used")
-	}
 	testenv.MustHaveExec(t)
 
 	// Test that mheap.sysAlloc handles collisions with other