Add testing infrastructure for slang

[fischeti]

I added another testing repository in tests/pickle to test out multiple SystemVerilog options. The testing is done on multiple levels:

Unit tests basic.rs

Just checks that parsing is sucessful or proper error is returned when broken SV code is provided.

Refactoring tests pickle.rs

Tests renaming feature of --prefix/--suffix as well as file filtering with --top

CLI regression cli_regression.rs

Existing test framework to check the emitted output against regression (to be enabled)

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This is part 2 of 3 in a stack made with GitButler:

•  3  Release flow adaptions for slang #271
•  2  Add testing infrastructure for slang #272 👈
•  1  Add slang bindings and pickle command #264