Narrow phpstan types in assertInstanceOf* AbstractTestCase methods

[staabm]

before this PR, PHPStan did not narrow the type of the arg given to e.g. assertInstanceOfCarbon

example taken from ToArrayTest:

        \PHPStan\dumpType($date); // line 100
        $this->assertInstanceOfCarbon($date ?? null);
        \PHPStan\dumpType($date); // line 102

before this PR

------ ------------------------------------------ 
  Line   ToArrayTest.php                           
 ------ ------------------------------------------ 
  100    Dumped type: Carbon\CarbonInterface|null  
  102    Dumped type: Carbon\CarbonInterface|null  
 ------ ------------------------------------------ 

after this PR

------ ------------------------------------------ 
  Line   ToArrayTest.php                           
 ------ ------------------------------------------ 
  100    Dumped type: Carbon\CarbonInterface|null  
  102    Dumped type: Carbon\CarbonInterface
 ------ ------------------------------------------ 

PHPStan now can narrow the non-null part in line 102, which will help preventing false-positives on higher PHPStan levels