Fix invalid returns in class getter's lazy evaluation blocks (#15364)

Using `return` in a class getter will immediately return from the
generated method, never setting the class variable to the returned
value, which is kept nil and so the initializer code is called
repeatedly instead of being called _once_ then cached.

spec/std/socket/spec_helper.cr

@@ -2,7 +2,9 @@ require "spec"
 require "socket"
 
 module SocketSpecHelper
-  class_getter?(supports_ipv6 : Bool) do
+  class_getter?(supports_ipv6 : Bool) { detect_supports_ipv6? }
+
+  private def self.detect_supports_ipv6? : Bool
     TCPServer.open("::1", 0) { return true }
     false
   rescue Socket::Error

spec/support/time.cr

@@ -72,7 +72,9 @@ end
     # Enable the `SeTimeZonePrivilege` privilege before changing the system time
     # zone. This is necessary because the privilege is by default granted but
     # disabled for any new process. This only needs to be done once per run.
-    class_getter? time_zone_privilege_enabled : Bool do
+    class_getter?(time_zone_privilege_enabled : Bool) { detect_time_zone_privilege_enabled? }
+
+    private def self.detect_time_zone_privilege_enabled? : Bool
       if LibC.LookupPrivilegeValueW(nil, SeTimeZonePrivilege, out time_zone_luid) == 0
         raise RuntimeError.from_winerror("LookupPrivilegeValueW")
       end