Improve ASCII Diffing for excluded tests

In `generate_runt_configs.py`:
# these cases are correct, but our ASCII diffing isn't good enough
# for us to know they are the same
xfailed = ["examples/serv/serv_regfile.tx",
"tests/adders/adder_d1/wait_and_add_correct.tx",
"tests/fifo/fifo.tx",
"tests/fifo/push_pop_identity_ok.tx",
"tests/wishbone/wishbone.tx"]

Start in generate_runt_configs.py and inspect the ASCII diffing logic alongside the xfailed list. Compare the listed cases to determine why equivalent outputs are treated as different; done means the diffing recognizes these correct cases as equal without requiring the exclusions.

Written by the indexing model from the issue text.