Add YAML pecl module to php container

What about running all core tests inside a patched test runner, just to be sure?

I was just wondering why the YAML extension is only added here for 5.5 and 5.6 and the conclusion I came to was because 1) it only makes sense to add for versions that are supported on Drupal 8 and 2) Maybe there is no version of the extension available yet for PHP 7? Is that about right?

For future reference, it seems like some work has been done on it ( https://github.com/php/pecl-file_formats-yaml/tree/php7 ) but I doubt there's any easy package for install.

Oh and hi btw @neclimdul :) Long time no see ... hope you're well.

Mostly #2. There doesnt yet exist a stable yaml extention for php7.

And... now we need one without it also.
#2597762: Add a new testbot with (only) Symfony yaml parser (not the pecl one)

Moved to DrupalCI Environments.

This now exists at the container level, and should be removed from the testrunner.