LibWeb: Add some tests that exercise the HTTP disk cache

Our HTTP disk cache is currently manually tested against various sites.
This patch adds some tests to cover various scenarios, including non-
cacheable responses, expired responses, and revalidation.

In order to ensure we hit the disk cache in RequestServer, we must
disable the in-memory cache in WebContent.

Libraries/LibWeb/Fetch/Fetching/Fetching.cpp

@@ -2848,6 +2848,11 @@ void set_http_cache_enabled(bool const enabled)
     g_http_cache_enabled = enabled;
 }
 
+bool http_cache_enabled()
+{
+    return g_http_cache_enabled;
+}
+
 void clear_http_cache()
 {
     HTTPCache::the().clear_cache();