[1.8.x] Fixed #24197 -- Added clearing of staticfiles caches on settings changes during tests

Cleared caching in staticfiles_storage and get_finder when
relevant settings are changed.

Backport of 2730dad0d7 from master

django/test/signals.py

@@ -156,3 +156,24 @@ def root_urlconf_changed(**kwargs):
         from django.core.urlresolvers import clear_url_caches, set_urlconf
         clear_url_caches()
         set_urlconf(None)
+
+
+@receiver(setting_changed)
+def static_storage_changed(**kwargs):
+    if kwargs['setting'] in {
+        'STATICFILES_STORAGE',
+        'STATIC_ROOT',
+        'STATIC_URL',
+    }:
+        from django.contrib.staticfiles.storage import staticfiles_storage
+        staticfiles_storage._wrapped = empty
+
+
+@receiver(setting_changed)
+def static_finders_changed(**kwargs):
+    if kwargs['setting'] in {
+        'STATICFILES_DIRS',
+        'STATIC_ROOT',
+    }:
+        from django.contrib.staticfiles.finders import get_finder
+        get_finder.cache_clear()