Temporary disable some send tests on nightly

Aparently there is a regression in the compiler and sync detection does not work correctly

Author: khvzak

tests/send.rs

@@ -47,6 +47,7 @@ fn test_userdata_multithread_access_send_only() -> Result<()> {
     Ok(())
 }
 
+#[rustversion::stable]
 #[test]
 fn test_userdata_multithread_access_sync() -> Result<()> {
     let lua = Lua::new();

tests/userdata.rs

@@ -957,6 +957,7 @@ fn test_nested_userdata_gc() -> Result<()> {
 }
 
 #[cfg(feature = "userdata-wrappers")]
+#[rustversion::stable]
 #[test]
 fn test_userdata_wrappers() -> Result<()> {
     #[derive(Debug)]