[Tests] Define ~SettingsCache in the benchmark mock

The mock includes cache_settings.h, which declares the destructor out of line,
so the vtable emitted for the mock SettingsCache references a definition that
never exists and picture_loader_benchmark_test fails to link. Default it: the
real destructor only frees release channels, and the mock creates none.

tests/picture_loader_benchmark/settings_cache_mock.cpp

@@ -144,6 +144,10 @@ SettingsCache::SettingsCache()
     loadPaths();
 }
 
+// The real destructor deletes the release channels; this mock never creates any,
+// but the vtable still needs the definition.
+SettingsCache::~SettingsCache() = default;
+
 void SettingsCache::loadPaths()
 {
     QString dataPath = getDataPath();