PIFO trees: better testing

[anshumanmohan]

This PR does the same treatment for PIFO trees that #1762 and #1763 do for FIFOs and PIFOs. That is, it:

• Generates a .data file automatically.
• Parses the .data file into Python and "runs" the PIFO tree regime as usual. Generates an .expect file.
• And then these automatic .data and .expect files can be used by runt to test the eDSL code as usual.

You will need:

python calyx-py/calyx/queue_data_gen.py > calyx-py/test/correctness/pifo_tree.data

and

python calyx-py/calyx/queue_data_gen.py | python calyx-py/calyx/pifotree_oracle.py > calyx-py/test/correctness/pifo_tree.expect

This PR continues the OO style introduced in #1763: creating the tree is a cinch because a PIFO is just an object that manages two queues, and a "tree" just happens to manage PIFOs and not FIFOs.