Introduce EditingUtilitiesTest.cpp as a home of nextPositionOf() test

Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/KeyframeEffect.idl',
             'animation/EffectModel.idl',
             'animation/AnimationEffectReadOnly.idl',
@@ skipping 3753 matching lines @@
             'dom/RangeTest.cpp',
             'dom/ScriptRunnerTest.cpp',
             'dom/StyleElementTest.cpp',
             'dom/TreeScopeStyleSheetCollectionTest.cpp',
             'dom/TreeScopeTest.cpp',
             'dom/shadow/ComposedTreeTraversalTest.cpp',
             'editing/CompositionUnderlineRangeFilterTest.cpp',
             'editing/EditingStrategyTest.cpp',
             'editing/EditingTestBase.cpp',
             'editing/EditingTestBase.h',
+            'editing/EditingUtilitiesTest.cpp',
             'editing/FrameSelectionTest.cpp',
             'editing/GranularityStrategyTest.cpp',
             'editing/InputMethodControllerTest.cpp',
             'editing/iterators/CharacterIteratorTest.cpp',
             'editing/iterators/SearchBufferTest.cpp',
             'editing/iterators/TextIteratorTest.cpp',
             'editing/markers/DocumentMarkerControllerTest.cpp',
             'editing/PositionTest.cpp',
             'editing/SurroundingTextTest.cpp',
             'editing/VisiblePositionTest.cpp',
@@ skipping 72 matching lines @@
             'style/OutlineValueTest.cpp',
             'style/SVGComputedStyleTest.cpp',
             'svg/UnsafeSVGAttributeSanitizationTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }