tests.support: Use kwargs overrides in get_settings()

This avoids harcoding test-specific overrides, and makes it easy to
setup a settings dictionary based on DEFAULT_CONFIG for testing.
Because you can trust Pelican to use settings based on DEFAULT_CONFIG,
you are free to go about using:

  settings[my_key]

instead of:

  settings.get(my_key, some_fallback)

or:

  if my_key in settings:
      ...

if you know that `my_key` is in DEFAULT_CONFIG.
Replace settings.get(key) with settings[key] for default settings

If a setting exists in DEFAULT_CONFIG, assume it will be there
(instead of checking and/or providing a local default).  The earlier
code was split between the two idioms, which was confusing.