arbitrary_source_item_ordering should ignore test modules

[alex-semenyuk]

Close #14570

The test is provided here but it's passed without //@compile-flags: --test

changelog: [arbitrary_source_item_ordering]: should ignore test modules

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[samueltardieu]

I'll handle both #14585 and #14580.
r? @samueltardieu

[samueltardieu]

Thanks for the contribution. However, in the future it may be great not to rush into issues marked with the "good first issue" label, those are identified in order to make it easier for beginners who want to make their first contribution to Clippy to start.

The PR looks good, but there is no test showing that it doesn't impact the rest of the lint. Could you please add the very same change you did in ordering_good.rs to selective_ordering.rs? This would check that the content of the module under #[cfg(test)] is still linted (as it will show that B should be declared after A). That would require an error marker for the ord_within test variant.