[5.4] Add assertSeeText() and assertDontSeeText() to TestResponse

[stidges]

This pull request adds two new assertions to the TestResponse class: assertSeeText() and assertDontSeeText().

Motivation

These assertions are really helpful when asserting against HTML responses, as visual design of the page shouldn't make a test pass or fail. Given the following HTML:

<strong>Views:</strong> 2

I would currently test this using the following assertion:

$response->assertSee('<strong>Views:</strong> 2');

But whenever a designer thinks it would look better if it was also italic, or add some extra CSS classes, my test would break. All I really care about is that the views are shown on the page. The assertSeeText() assertion solves this issue:

$response->assertSeeText('Views: 2');