fix and update test suite

[Elhebert]

This PR fixes a failing tests, removes all deprecation and warning from the test suite. It also makes the code more consistent by adding return types to every methods.

Other improvements made are:

• Using assertCount(x, y) instead of assertEquals(x, count(y)) for clarity.
• Using MyClass::class rather than \\Namespace\\MyClass when asserting the instance of a class.
• Removing dead code.
• Improving the general code style for more consistency.
• Changing the assertRegExp to assertMatchesRegularExpression as the former is deprecated is PHPUnit 10+.
• Remove the require statement in favor of proper namespace imports.

I would also be happy to add a GitHub workflow to replace the travis pipeline if this is something you’d like to do.