tests: address review on picture loader benchmark

- Sandbox via unique app/org names plus Linux-only XDG redirection, derive
  the warm-cache probe and data path from SettingsCache, and bail out when the
  temporary sandbox cannot be created.
- Run each pass with a fresh worker and shut workers down before reading the
  429 counters, so the redirect cache is persisted and no worker thread can
  outlive the stack-local counters or the installed message handler.
- Make s_activeCounters atomic and always forward log output when the previous
  handler is the built-in (nullptr) one.
- Validate --timeout-min, scale the cached-pass budget with --count, honour the
  first --url as the stress template, and add the missing trailing newlines.
- Zero-initialise SettingsCache members so the benchmark mock cannot
  dereference an indeterminate pointer.

cockatrice/src/client/settings/cache_settings.h

@@ -58,33 +58,33 @@ signals:
     void themeChanged();
 
 private:
-    QSettings *settings;
-    ShortcutsSettings *shortcutsSettings;
-    CardDatabaseSettings *cardDatabaseSettings;
-    ServersSettings *serversSettings;
-    MessageSettings *messageSettings;
-    GameFiltersSettings *gameFiltersSettings;
-    LayoutsSettings *layoutsSettings;
-    DownloadSettings *downloadSettings;
-    RecentsSettings *recentsSettings;
-    CardOverrideSettings *cardOverrideSettings;
-    DebugSettings *debugSettings;
-    CardCounterSettings *cardCounterSettings;
-    TabsSettings *tabsSettings;
-    SoundSettings *soundSettings;
-    GameSettings *gameSettings;
-    ChatSettings *chatSettings;
-    CacheStorageSettings *cacheStorageSettings;
-    UpdatesSettings *updatesSettings;
-    PersonalSettings *personalSettings;
-    CardsDisplaySettings *cardsDisplaySettings;
-    InterfaceSettings *interfaceSettings;
-    DeckEditorSettings *deckEditorSettings;
-    PathsSettings *pathsSettings;
-    VisualDeckStorageSettings *visualDeckStorageSettings;
-    AppearanceSettings *appearanceSettings;
-    NetworkSettings *networkSettings;
-    CommanderBracketSettings *commanderBracketSettings;
+    QSettings *settings = nullptr;
+    ShortcutsSettings *shortcutsSettings = nullptr;
+    CardDatabaseSettings *cardDatabaseSettings = nullptr;
+    ServersSettings *serversSettings = nullptr;
+    MessageSettings *messageSettings = nullptr;
+    GameFiltersSettings *gameFiltersSettings = nullptr;
+    LayoutsSettings *layoutsSettings = nullptr;
+    DownloadSettings *downloadSettings = nullptr;
+    RecentsSettings *recentsSettings = nullptr;
+    CardOverrideSettings *cardOverrideSettings = nullptr;
+    DebugSettings *debugSettings = nullptr;
+    CardCounterSettings *cardCounterSettings = nullptr;
+    TabsSettings *tabsSettings = nullptr;
+    SoundSettings *soundSettings = nullptr;
+    GameSettings *gameSettings = nullptr;
+    ChatSettings *chatSettings = nullptr;
+    CacheStorageSettings *cacheStorageSettings = nullptr;
+    UpdatesSettings *updatesSettings = nullptr;
+    PersonalSettings *personalSettings = nullptr;
+    CardsDisplaySettings *cardsDisplaySettings = nullptr;
+    InterfaceSettings *interfaceSettings = nullptr;
+    DeckEditorSettings *deckEditorSettings = nullptr;
+    PathsSettings *pathsSettings = nullptr;
+    VisualDeckStorageSettings *visualDeckStorageSettings = nullptr;
+    AppearanceSettings *appearanceSettings = nullptr;
+    NetworkSettings *networkSettings = nullptr;
+    CommanderBracketSettings *commanderBracketSettings = nullptr;
 
     QString themeName;