…ethod

PHPUnit 9.4.0 introduced the new `Assert::assertObjectEquals()` method.

This commit:
* Adds two traits with the same name.
    One to polyfill the method when not available in PHPUnit.
    The other to allow for `use`-ing the trait in PHPUnit versions in which the method is already natively available.
* Adds an `InvalidComparisonMethodException` exception class.
    _PHPUnit natively throws a range of different exceptions._
    _The polyfill included in this library throws one exception type - the `InvalidComparisonMethodException` - with a range of different messages._
* Logic to the custom autoloader which will load the correct trait depending on the PHPUnit version used.
* Adds tests.

As the polyfill contains logic to match the PHPUnit native implementation as closely as possible, while still being PHP and PHPUnit cross-version compatible, extensive unit tests have been added to ensure the behaviour of the polyfill matches that of the original function, with the exception of the _return type verification_.

As return types were not available in PHP prior to PHP 7.0, the return type verification as done in the PHPUnit native implementation, has been replaced by a verification that the _returned value_ is of the required type.
This provides the same safeguard as the PHPUnit native implementation, but in a PHP cross-version compatible manner.

Note: the method uses `static::` to call the PHPUnit native functionality. This allows for existing method overloads in a child class of the PHPUnit native `TestCase` to be respected.

Includes:
* Adding information on the new polyfill to the README.
* Adding the new polyfill to the existing `TestCases` classes.
* Adding a few select exceptions to the PHPCS ruleset for the fixtures used in the tests.

Refs:
 * sebastianbergmann/phpunit#4467
 * sebastianbergmann/phpunit#4707
 * sebastianbergmann/phpunit@1dba8c3
 * sebastianbergmann/phpunit@6099c5e