Auto merge of #111269 - clubby789:validate-fluent-variables, r=davidtwco

Validate fluent variable references in tests

Closes #101109

Under `cfg(test)`, the `fluent_messages` macro will emit a list of variables referenced by each message and its attributes. The derive attribute will now emit a `#[test]` that checks that each referenced variable exists in the structure it's applied to.

Author: bors