Add helper classes for shortcode testing

[mpdude]

This PR addresses the shortcomings outlined in #12 and #13, and probably replaces #10 and #18 as well.

1. Deprecate the \Webfactory\ShortcodeBundle\Tests\Functional\ShortcodeTest class.

• It was not available through the regular autoload mechanism (required extra trickery),
• depended on undeclared dependencies (BrowserKit and DomCrawler),
• came without tests,
• suggested users to do/prefer full-scale Application Testing (using WebTestCase) and
• worked only through the means of (ab-)using the Shortcode Guide.

2. Provide alternate means of testing shortcode processing

• Encourage users to do plain/direct unit testing (or integration testing, when necessary) of their shortcode handlers and/or shortcode handling controllers through new sections in the README. This gives best (most direct) control of input parameters.
• Add the public \Webfactory\ShortcodeBundle\Test\ShortcodeDefinitionTestHelper service that can be used in Kernel-based (functional) tests to verify a given shortcode name is known, the controller can be resolved etc.
• Add a \Webfactory\ShortcodeBundle\Test\EndToEndTestHelper class that simplifies full end-to-end (from content to processed content) functional testing, including round-trips through Symfony's FragmentHandler for controller-based shortcodes.

Fixes #12, fixes #13, closes #10, closes #18, closes #24.

[MalteWunsch]

Good job 💚

[MalteWunsch]

I've noticed these double underscores several times and wonder if I've missed some meaning?

[mpdude]

No, somehow they keep reappearing when I run PHP-CS-Fixer.

[MalteWunsch]

I'm not entirely happy for our tests to depend on this external PlaceholderHandler class. But it is officially mentioned in thunderer's README, so 👍