[Prototype] Lit testing for the Unified Compiler

[mudit2812]

Context:
We're considering using proper lit testing for the unified compiler. This PR is a proof of concept for what such tests may look like.

Description of the Change:

• Add lit tests for combine-global-phases
  • These are just a copy of the filecheck tests in test_xdsl_combine_global_phases.py
• Minor update to lit.cfg.py to allow MLIR files to be usable as lit tests
• Updated how we set up CATALYST_XDSL_UNIVERSE. The class expects the dialect and transform dictionaries to have functions that return the respective dialects/transforms as values rather than the dialects/transforms themselves.

Benefits:

• With a fully set up language server (as described here), writing lit tests was a really nice process. The IDE immediately raised errors if any of the operations in the IR were defined incorrectly with good error messages, making it easy to figure out what to fix.
• Syntax highlighting 😍

Some notes:

• The // ----- separator between tests is necessary so that lit can split of the file into separate tests properly.
• The command to run tests is:

  lit test_combine_global_phases.mlir
  

• The RUN comment at the top is used by lit to execute filecheck
  • We use xdsl-opt to apply the pass to the input file (specified with %s)
  • The output is piped to FileCheck, which compares the output of xdsl-opt with the input file

Possible Drawbacks:

• I personally found the error messages for test failures a bit harder to parse. While the general content of the errors was very similar, the verbosity and formatting of the message made it a lot less readable

Related GitHub Issues:

[sc-109712]