Add spec to manual specs (too slow)

spec/manual/hash_large_spec.cr

@@ -0,0 +1,8 @@
+require "spec"
+
+it "creates Hash at maximum capacity" do
+  # we don't try to go as high as Int32::MAX because it would allocate 18GB of
+  # memory in total. This already tests for Int32 overflows while 'only' needing
+  # 4.5GB of memory.
+  Hash(Int32, Int32).new(initial_capacity: (Int32::MAX // 4) + 1)
+end