tests: coverage round 4 (footage, IPC, input immediate, project/factory)

- footage_test: static describe/loop-mode helpers, stream mapping,
  Value() job generation incl. proxy attachment, data roles, reprobe
  via seeded metadata cache
- render_workerpool_ipc_test: SharedMemoryRegion, FrameSlotPool
  cross-mapping handoff, IpcMessage NDJSON round trips, RenderWorkerPool
  rejection paths
- node_inputimmediate_test: NodeInputImmediate raw API, SetValueAtTime,
  interpolation (linear/hold/bezier) for float/vec/color/rational
- project_factory_test: Project settings/cache modes/save-load/signals,
  NodeFactory creation and menus

Also fixes Project::cache_path() returning the default cache instead of
a configured custom cache path (inverted branch, found by the tests)

tests/gtest/CMakeLists.txt

@@ -59,6 +59,10 @@ add_executable(olive-gtest
   node_core_test.cpp
   clip_traverser_test.cpp
   audio_manager_viewer_test.cpp
+  footage_test.cpp
+  render_workerpool_ipc_test.cpp
+  node_inputimmediate_test.cpp
+  project_factory_test.cpp
   timeline_marker_test.cpp
   undo_stack_test.cpp
   plugin_support_test.cpp