Fix ODR violation that can break WTF::String pretty printing in tests.

Source/wtf/wtf.gypi

 # For GN compatibility, this file can not have any conditions.
 {
     'variables': {
         'wtf_files': [
             'ASCIICType.h',
             'AddressSanitizer.h',
             'AddressSpaceRandomization.cpp',
             'AddressSpaceRandomization.h',
             'Alignment.h',
             'ArrayBuffer.cpp',
@@ skipping 207 matching lines @@
             'MathExtrasTest.cpp',
             'OptionalTest.cpp',
             'PartitionAllocTest.cpp',
             'RefPtrTest.cpp',
             'SaturatedArithmeticTest.cpp',
             'StringExtrasTest.cpp',
             'StringHasherTest.cpp',
             'TemporaryChangeTest.cpp',
             'TreeNodeTest.cpp',
             'VectorTest.cpp',
-            'testing/WTFTestHelpersTest.cpp',
+            'testing/WTFTestPrintersTest.cpp',
             'text/AtomicStringTest.cpp',
             'text/CStringTest.cpp',
             'text/StringBufferTest.cpp',
             'text/StringBuilderTest.cpp',
             'text/StringImplTest.cpp',
             'text/StringOperatorsTest.cpp',
             'text/TextCodecReplacementTest.cpp',
             'text/TextCodecUTF8Test.cpp',
             'text/WTFStringTest.cpp',
         ],
         'wtf_unittest_helper_files': [
-            'testing/WTFTestHelpers.cpp',
-            'testing/WTFTestHelpers.h',
-            'testing/WTFUnitTestHelpersExport.h',
+            'testing/WTFTestPrinters.cpp',
         ],
     },
 }