feat: first step towards the test command

[a-frantz]

Describe the problem or feature in addition to a link to the issues.

As the wdl-engine interface is currently undergoing a refactor, this PR does not interact with wdl-engine at all. This PR simply defines the YAML which will be used for defining unit tests and does some basic parsing and logging as proof of concept.

stjude-rust-labs/rfcs#1

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[peterhuene]

This looks like a great first step towards the test command.

I have a few feedback comments and at least one "discussion" comment regarding the UX of the feature that we can easily punt to later if desired.

[a-frantz]

@peterhuene I believe that I've "rustified" this sufficiently. We are yielding iterators, avoiding redundant String allocs, and validating+collecting errors as they are encountered then reporting all of the problems with found tests.

I've also gone ahead and added some simple tagging and filtering of tests (--include-tag and --filter-tag CL args).

Lastly, I've made another change to where tests are looked for. In addition to test YAML being looked for at "sibling" locations (e.g. tools/foo.wdl and tools/foo.yaml) tests can also be nested under a sibling test/ directory (e.g. tools/foo.wdl and tools/test/foo.yml) The cli:matrix_combinations test shows an example:

tests/cli/test/matrix_combinations/inputs/
├── example.wdl
├── example.yaml
├── flag_filter.wdl
└── test
    └── flag_filter.yml

Test files still have to have the same basename as the WDL they are testing.

I believe the above addresses all the received feedback 🎉 thanks for the review! LMK if there are any other changes we want to get in this PR

[peterhuene]

Looks great, just two nit comments!