tester: enable prop_multiple_equally_applicable rewriter check

Enable the `prop_multiple_equally_applicable` rewriter assertion in the
integration tester. This check panics if a given expression has two
rules of the same priority applicable to it.

All tests pass with this enabled.

For performance reasons, I think it is best to keep this disabled by
default in the CLI.

Issue: conjure-cp#552

conjure_oxide/tests/generated_tests.rs

@@ -208,7 +208,7 @@ fn integration_test_inner(
         "Model before rewriting:\n\n{}\n--\n",
         model
     );
-    let model = rewrite_naive(&model, &rule_sets, false)?;
+    let model = rewrite_naive(&model, &rule_sets, true)?;
 
     tracing::trace!(
         target: "rule_engine_human",