Cleaning up tests

[darichey]

I think the tests in the crate could use some improvement. It's not clear what the difference is between test_data/general and test_data/parser, and parser tests are snapshot-based while lexer tests are not

I would like to propose the following:

• Replace the ad-hoc snapshot testing implementation with expect_test
• Make lexer tests snapshot-based
• Reorganize test_data like so:

  test_data/
  ├── parser/
  │   ├── success/
  │   │   ├── assert.nix
  │   │   ├── assert.expect
  │   │   ├── with.nix
  │   │   ├── with.expect
  │   │   └── ...
  │   └── failure/
  │       ├── missing_semi.nix
  │       ├── missing_semi.expect
  │       └── ...
  └── lexer/
      ├── success/
      │   └── ...
      └── failure/
          └── ...

• Give each test file a meaningful name to indicate what is being tested (so not just 1.nix)

I am willing to implement the above, but I wanted to get some feedback before starting!

[oberblastmeister]

This is very much needed! I will 100% accept all of these changes.