CRM_Extension_System - Only handle `test.*` extensions on headless test sites

CRM_Extension_System - Only handle `test.*` extensions on headless test sites

The headless test suite includes integration tests for correct handling of
the extension installation/removal process. This involves some
dummy extensions. Unfortunately, the dummy extensions are often picked up on
live deployments -- where they're a bit distracting/annoying.

Before
------

The `test.*` dummy extensions appear as available in all environments,
including headless tests and live sites.

After
-----

The `test.*` dummy extensions are only available in the headless test
environment.