Fix: SocketSpecHelper.supports_ipv6?

The initializer block is now captured, and we can't return from a
captured block. This outlines that the previous commit is a BREAKING
CHANGE!

The `class_getter?` version used skip over the intent to cache the
result in `@@supports_ipv6` (it returned from the generated function,
not from the block), so whenever IPv6 was supported every test was
creating yet-another TCPServer (oops).

spec/std/socket/spec_helper.cr

@@ -2,11 +2,18 @@ require "spec"
 require "socket"
 
 module SocketSpecHelper
-  class_getter?(supports_ipv6 : Bool) do
-    TCPServer.open("::1", 0) { return true }
-    false
-  rescue Socket::Error
-    false
+  @@supports_ipv6 : Bool?
+
+  def self.supports_ipv6? : Bool
+    unless (value = @@supports_ipv6).nil?
+      return value
+    end
+    begin
+      TCPServer.open("::1", 0) { return @@supports_ipv6 = true }
+      @@supports_ipv6 = false
+    rescue Socket::Error
+      @@supports_ipv6 = false
+    end
   end
 end