Add painfully slow to create hash at max capacity

spec/std/hash_spec.cr

@@ -1393,6 +1393,10 @@ describe "Hash" do
     hash.@indices_size_pow2.should eq(12)
   end
 
+  it "creates with max capacity", slow: true do
+    Hash(Int32, Int32).new(initial_capacity: Int32::MAX)
+  end
+
   describe "#rehash" do
     it "rehashes" do
       a = [1]