tests: coverage for small utility widgets (66 cases)

- slider: SliderBase/FloatSlider/IntegerSlider mapping, clamping,
  display transforms, drag math, tristate, label substitution
- layouts: FlowLayout wrap math, ColumnedGridLayout placement
- combos: all standard combos, node combo box, color label combo
- misc: menu, file/path fields, toolbar, color button/wheel widgets,
  bezier, resizable scrollbar, hand-movable view, node value tree,
  pixel sampler, collapse button, clickable/focusable labels

tests/gtest/CMakeLists.txt

@@ -128,6 +128,10 @@ add_executable(olive-gtest
   dialog_editing_test.cpp
   dialog_misc_test.cpp
   dialog_export_test.cpp
+  widget_slider_test.cpp
+  widget_layout_test.cpp
+  widget_combos_test.cpp
+  widget_misc_test.cpp
 )
 
 find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Test)