Add attribute does not contain assertion

[ziadoz]

I've found I sometimes need the opposite of assertAttributeContains() in tests.

For example, I've often got elements where Bootstrap classes are being toggled in JS:

<ul class="list-inline">
    <li class="list-inline-item main d-none">Foo</li>
    <li class="list-inline-item sub">Bar</li>
</ul>

Currently I have to assert all the classes on the second <li>, even though I'm only interested in d-none:

$browser->assertAttributeContains('li:nth-child(1)', 'class', 'd-none');
$browser->assertAttribute('li:nth-child(2)', 'class', 'list-inline-item sub');

After this PR its clearer what's being tested:

$browser->assertAttributeContains('li:nth-child(1)', 'class', 'd-none');
$browser->assertAttributeDoesNotContain('li:nth-child(2)', 'class', 'd-none');