Step Unit Test Infrastructure

[donn]

• Created step unit testing infrastructure, relying on specially-laid out folders that provide input data with the ability to add a per-step input data preprocessor and output
  • Requires --all-steps flag to be passed to pytest
  • Created submodule to host step tests
  • Added step to Nix build that utilizes the step unit testing infrastructure
• Created method openlane/common/misc.py::Path::startswith
• Added a new internal subcommand, openlane.steps create-test, which creates a flat reproducible that can be more easily added as a step unit
• Expanded test coverage to openlane/steps/checker.py
• Expanded openlane/common/generic_dict.py::copy_recursive to support dataclasses
• Fixes and tweaks for step reproducibles:
  • openlane.steps create-reproducible has new flag, --no-include-pdk, which excludes PDK files from reproducibles and make any reference to them utilize pdk_dir::
  • Slight internal rework for openlane/steps/step.py::Step::create_reproducible to support flattened file structures suitable for testing
  • Reproducibles now no longer include views of the design not explicitly declared in step .inputs
• Metric design__latch__count renamed to design__inferred_latch__count
• Reworked Nix derivations for the OpenLane shell, OpenLane, and the Docker image
• Fixed bug in Verilator.Lint where inferred latch warnings were not properly processed
• Fixed bug in openlane/config/config.py::__load_dict where pdkpath was not being set in some scenarios