Update test ignore annotations for aarch64 (#1643)

Looks like everything is in general passing now so it's probably time to
close #1521 and all other remaining tests that are failing are
classified under new more focused issues.

Closes #1521

tests/all/custom_signal_handler.rs

@@ -87,7 +87,6 @@ mod tests {
     }
 
     #[test]
-    #[cfg_attr(target_arch = "aarch64", ignore)] // FIXME(#1521)
     fn test_custom_signal_handler_single_instance() -> Result<()> {
         let engine = Engine::new(&Config::default());
         let store = Store::new(&engine);