Experimenting with more structured ways to handle command-line input/output in Rust

Improve `fluent_embed_derive` test suite

Makes everything much cleaner using rstest and pretty_assertions, in preparation for more tests being added.

Created by  finchie  on July 30, 2024
6ABVDTXZOHVUDZDKDQS256F74LFIMM5DO3OZWHKRXZBUTPII4WAQC
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