[5.0] Fix with() and page assertions

[staudenmeir]

Using with() in combination with a page can break tests:

class TestPage extends Page
{
    public function assert(Browser $browser)
    {
        $browser->assertSee('Page Title');
    }
}

$this->browse(function (Browser $browser) {
    $browser->visit(new TestPage)
        ->with('.prefix', function (Browser $browser) {
            // 
        });
});

Dusk executes the page's assertions in visit() and in with(), but the latter one doesn't always work. In this example, the page title is not available inside the .prefix scope.

on() should only execute the assertions when being called directly or from visit(), but not when being called from with().

Fixes #618.

---

A more elegant but breaking refactoring for Dusk 6.0 could look like this:

public function on($page, $assert = true)
{
    $this->page = $page;

    // Here we will set the page elements on the resolver instance, which will allow
    // the developer to access short-cuts for CSS selectors on the page which can
    // allow for more expressive navigation and interaction with all the pages.
    $this->resolver->pageElements(array_merge(
        $page::siteElements(), $page->elements()
    ));

    if ($assert) {
        $page->assert($this);
    }

    return $this;
}

public function onWithoutAssert($page)
{
    return $this->on($page, false);
}