Fix "php_unit_test_case_static_method_calls" CS

[mvorisek]

originally discussed in #9303 (comment)

This is non-functional change only, but I belive calling TestCase::assertXxx static methods as non-static like $this->assertXxx() should be replaced with self::assertXxx() because static methods should be simply called statically.

Later, we should require this using PHPStan across the whole project, for now, let's require this using PHP CS Fixer for (known) TestCase methods only.

[alecpl]

I generally don't like this idea. Whenever I have an object instance I prefer to use it instead of a static call. E.g.

$var1 = $object->method();
$var2 = $object->staticMethod();

looks better than

$var1 = $object->method();
$var2 = SomeClass::staticMethod();

Maybe if a class has static methods only... So, probably TestCase is such a case, but everything inheriting from it should be using static methods (and properties) only too.

[mvorisek]

I would say you are used to that, but it is definitely less readable, as one call needs an instance, the other not. With ->method(), you cannot tell just by looking at the code. Also, PHP CS Fixer needs :: to be used to correctly make anonyous functions static which are optimized better as they do not need to be rebound before a call is made.

Let's land this for the PHPUnit static methods. It will be checked by CI, thus we can be sure only one coding style will be used.

[mvorisek]

PR is done and it fixes most of the ^Dynamic call to static method .+\. phpstan errors.

[mvorisek]

@alecpl can this PR be merged?