(REF) Convert YAML fixtures to JSON fixtures. Reset symfony/yaml dependency.

(REF) Convert YAML fixtures to JSON fixtures. Reset symfony/yaml dependency.

Overview
--------

The test suite includes some tests that read fixture data files (i.e. YAML files). This patch
provides a safer way to address a dependency-management issue with parsing YAML files.

Before
------

* In the last public release (`5.14`), the `symfony/yaml` package was not included with `civicrm-core`.
  However, it was loaded during testing by virtue of running `phpunit6`.
* In the `5.15` RC code, `symfony/yaml` was added -- so that the test suite worked in `phpunit6`.
  However, this also added `symfony/yaml` to the general runtime distribution - which is liable to create conflicts
  with Civi extensions and CMS modules that also bundle `symfony/yaml`. (It's a popular package.)

After
-----

* The test suite no longer uses YAML, and the build no longer requires `symfony/yaml`. Instead, it uses JSON.