Tests/RestrictedClasses: compatibility with PHPCS 4.x
Tests/RestrictedClasses: compatibility with PHPCS 4.x

PHPCS 4.x is set up to use PHPUnit 8+, including using the `void` return type for the `setUp[BeforeClass]()` and `tearDown[AfterClass]()` fixtures.

This commit makes our tests compatible with both PHPCS 3.x as well as 4.x and with PHPUnit < 8 and 8+.

This is done by:
* Renaming the fixtures in use in our test suite to use a different method name (only this class).
* And using the PHPUnit `@before` and `@after` annotations, which translate to `setUp()` and `tearDown()` to get round the `void` return type requirement.

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