Lexer tests should check that their output is as expected

[PatchMixolydic]

Several lexer tests currently just emit a diagnostic and hope that you'll think to use --show-output:

https://github.com/PatchMixolydic/laria/blob/13718184cacd584b32b7532749c46c1e8226c735/laria_compiler/src/script/parser/tests.rs#L33-L53

These tests should check their output themselves.

[PatchMixolydic]

These should also probably receive integration tests.