test: wire the handler property tests and make the proptest recipe run them

handler_proptest.rs was committed without ever being declared as a module, and it imported a list_size strategy that did not exist, so it never compiled. The proptest recipe filtered on a test name substring that matched no test.

Declare the module, add the two strategies it needs, and pass both property test filters to libtest so the recipe runs the ui and handler property tests.

src/handlers/handler_test_utils.rs

@@ -1,6 +1,6 @@
 #[cfg(test)]
 #[macro_use]
-mod test_utils {
+pub mod test_utils {
   #[macro_export]
   macro_rules! simple_stateful_iterable_vec {
     ($name:ident) => {
@@ -411,4 +411,16 @@ mod test_utils {
       );
     };
   }
+
+  pub mod proptest_helpers {
+    use proptest::prelude::*;
+
+    pub fn list_size() -> impl Strategy<Value = usize> {
+      1usize..100
+    }
+
+    pub fn text_input_string() -> impl Strategy<Value = String> {
+      "\\PC{0,50}"
+    }
+  }
 }