Add comprehensive syncback roundtrip and clean mode stress tests

- Completed the implementation of roundtrip tests to validate the syncback process, ensuring that building a project, syncing back, and rebuilding produces semantically identical outputs.
- Created a new utility module for roundtrip testing, including functions for running builds and asserting directory equality.
- Developed clean mode stress tests to verify that syncing back to a dirty directory yields identical results to syncing back to a fresh directory, covering various mutation scenarios.
- Updated existing test modules to incorporate new test cases and ensure consistency in handling standalone scripts transitioning to directory format when children are added.

tests/rojo_test/mod.rs

@@ -1,4 +1,5 @@
 pub mod internable;
 pub mod io_util;
+pub mod roundtrip_util;
 pub mod serve_util;
 pub mod syncback_util;