Phester needs a way to create fixtures that can then be used by different test suites. Fixtures are defined in yaml files just like test suites, but they use the "fixture" field of the top level object to specify a unique name (while test suites use the "suite" field).

Notes:

• See T219883: Draft file format for phester test definitions for the structure of fixture files.
• All fixtures must be executed before any test suite
• Fixtures that are not needed by any test suite should not be executed
• Fixtures may depend on other fixtures. A dependency resolution algorithm (topological sort) is needed to determine execution order.
• Fixtures will eventually export sessions and variables for use by tests. That ability will however be added later, see T227885 and T227887.
• Fixtures must have a "setup" stanza
• Fixtures may have a "test" stanza for sanity checks