Stop Excluding Tests from Coding Standards

[kopepasah]

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Feature Request

Currently, tests are excluded from the coding standards check: https://github.com/xwp/stream/blob/develop/phpcs.xml.dist#L25

As more tests are being written, we either should not exclude these from this check or create a check to ensure standards are followed in tests as well.

[kopepasah]

Specifically relates to #1118, in regards to shorthand vs long form array syntax: https://github.com/xwp/stream/pull/1118#discussion_r458374497

[kopepasah]

Just adding a note here that I am currently working through adding this functionality, including ensuring current tests meet standards.