Add regression testing

[justinlubin]

Add a suite of regression tests that checks each of the benchmarks against the previous version of Smyth to see if the synthesis outputs change.

It would be useful to have modes for this feature that allow for:

• Checking to see if the synthesis output is the exact same string as before
• Checking to see if the synthesis output is the exact same AST as before
• Checking to see if the synthesis output is in the same ranked order as before