Separate ValueTest and StringValueTest into headers.

Change-Id: If326e6943a11f271ef939dc815a45ba0b4ee271c

test/unit/StringValueTest.h

@@ -0,0 +1,12 @@
+#include "Value.h"
+
+#include "gtest/gtest.h"
+
+namespace ntimpl {
+
+class StringValueTest : public ::testing::Test {
+ public:
+  NT_String& ToNT(StringValue& v) { return v; }
+};
+
+}  // namespace ntimpl