Add a new testbot with (only) Symfony yaml parser (not the pecl one)

Problem/Motivation

#2565025: Add YAML pecl module to php container
added YAML pecl module, but in order to actually test both environments,
we need one that also doesn't have it, so that we can test using the Symfony one also.

See #1920902-249: Add a Drupal Yaml wrapper so we can default to PECL Yaml component if it is available
@alexpott

We also agreed that in order to proceed with these change we need to have at least one more test environment that is using Symfony to read yaml files so that this common environment is at least tested.

Proposed resolution

Add a new testbot with (only) Symfony yaml parser

Remaining tasks

User interface changes

API changes

Data model changes