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.

pelican/tests/support.py

@@ -161,10 +161,14 @@ def locale_available(locale_):
         return True
 
 
-def get_settings():
+def get_settings(**kwargs):
+    """Provide tweaked setting dictionaries for testing
+
+    Set keyword arguments to override specific settings.
+    """
     settings = DEFAULT_CONFIG.copy()
-    settings['DIRECT_TEMPLATES'] = ['archives']
-    settings['filenames'] = {}
+    for key,value in kwargs.items():
+        settings[key] = value
     return settings