tests: coverage round 8 (serialization, folder, text/polygon, probe, render tail)

- node_save_load_test: Node Save/Load round trips for values, keyframes,
  hints, caches, connections, positions
- node_polygon_folder_test: Folder child management, PolygonGenerator
  rasterization/gizmos, TextGeneratorV1/V2
- footage_probe_test: real FFmpeg/OIIO probing of demo.mp4/img.png,
  metadata cache, footage state transitions
- render_tail_test: DynamicRenderer, color-context shader plumbing,
  PreviewAutoCacher pause/clear paths, DiskManager edges,
  AudioPlaybackCache segment I/O

Also fixes AudioPlaybackCache::WritePartOfSampleBuffer, found by the new
tests: zero padding was written via QFile::write(const char*) which
treats the buffer as a NUL-terminated string, so no padding bytes were
ever written; the write length was also computed from the segment end
instead of the range end, making WriteSilence() spin forever on an
empty buffer

tests/gtest/CMakeLists.txt

@@ -75,6 +75,10 @@ add_executable(olive-gtest
   node_distort_test.cpp
   node_filter_keying_test.cpp
   node_math_transition_test.cpp
+  node_save_load_test.cpp
+  node_polygon_folder_test.cpp
+  footage_probe_test.cpp
+  render_tail_test.cpp
   timeline_marker_test.cpp
   undo_stack_test.cpp
   plugin_support_test.cpp