webperf: Introduce 'templates' in test fixture and use for mwload
- Create a 'templates' key in the fixture that the test runner ignores.
- Move the existing 'expected_uncached_desktop' list there.
- Abstract another template list for 'expected_uncached_desktop_mwload' that is also used by the non-navtiming test.
- Add 'flatten()' utility method given the ref/merge ref/extend syntax only works for key-value objects (dict) in YAML, not for sequences (list).