Backfill some FX String unit tests for == and !=.

core/include/fxcrt/fx_string.h

Index: core/include/fxcrt/fx_string.h
diff --git a/core/include/fxcrt/fx_string.h b/core/include/fxcrt/fx_string.h
index a7cf2e1c16d0817dac55bb350d44082ef5322878..26909c84a481b99d97820a595ed40cf406c0f823 100644
--- a/core/include/fxcrt/fx_string.h
+++ b/core/include/fxcrt/fx_string.h
@@ -178,7 +178,10 @@ struct CFX_StringData {
 
     FX_CHAR		m_String[1];
 };
-class CFX_ByteString 
+
+// A mutable string with shared buffers using copy-on-write semantics that
+// avoids the cost of std::string's iterator stability guarantees.
+class CFX_ByteString
 {
 public:
     typedef FX_CHAR value_type;
@@ -595,7 +598,10 @@ struct CFX_StringDataW {
 
     FX_WCHAR	m_String[1];
 };
-class CFX_WideString 
+
+// A mutable string with shared buffers using copy-on-write semantics that
+// avoids the cost of std::string's iterator stability guarantees.
+class CFX_WideString
 {
 public:
     typedef FX_WCHAR value_type;