configurably allow `useless_vec` in tests

[llogiq]

This adds a àllow-useless-vec-in-test configuration which, when set to true will allow the useless_vec lint in #[test] functions and code within #[cfg(test)]. It also moves a is_in_test helper to clippy_utils.

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changelog: configurably allow [useless_vec] in test code

[rustbot]

r? @Jarcho

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[llogiq]

Jarcho seems to be busy recently, so perhaps r? @blyxyas

[blyxyas]

@bors r+

[bors]

:pushpin: Commit db40ab45dac590d2435367e359ff350675f9b4bc has been approved by blyxyas

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit db40ab45dac590d2435367e359ff350675f9b4bc with merge 5300d45877f09541463c692a6220985945d7a664...

[bors]

:broken_heart: Test failed - checks-action_test

[blyxyas]

@bors r+

[bors]

:pushpin: Commit 87efce4fa22f51075e1fd25904f757f783eebfdc has been approved by blyxyas

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit 87efce4fa22f51075e1fd25904f757f783eebfdc with merge e68fcb09734cc63ddbac7c0263a9d1b6e2549cd5...

[bors]

:sunny: Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test Approved by: blyxyas Pushing e68fcb09734cc63ddbac7c0263a9d1b6e2549cd5 to master...