Add option to automatically generate failing.llvm

[anvacaru]

• Changes to the test_conformance.py harness:
  1. add a flag --save-failing to run and save failing tests to failing.llvm
  2. When the flag is active, all the tests of a GST file will be executed, otherwise, it will raise at the first failing test.
• Regenerating failing.llvm

Before using the new flag, delete the contents of failing.llvm.

invocation example:

# make sure the --maxfail flag is not passed or it is set to a large number.
poetry run pytest src/tests/integration --verbose --durations=0 --dist=worksteal --numprocesses=8 --save-failing -k test_conformance.py