DocumentMove tests: compiler error due to -Wsign-compare

On GCC 4.9, the `documenttest.cpp` fails to compile in C++11 mode, as the MoveConstructor/Assignment tests contain left-over comparisons between signed and unsigned expressions:

````
In file included from ../../test/unittest/unittest.h:43:0,
                 from ../../test/unittest/documenttest.cpp:21:
../../thirdparty/gtest/include/gtest/gtest.h: In instantiation of ‘testing::AssertionResult testing::internal::CmpHelperGT(const char*, const char*, const T1&, const T2&) [with T1 = unsigned int; T2 = int]’:
../../test/unittest/documenttest.cpp:408:5:   required from ‘void DocumentMove_MoveAssignmentStack_Test<gtest_TypeParam_>::TestBody() [with gtest_TypeParam_ = rapidjson::MemoryPoolAllocator<>]’
../../test/unittest/documenttest.cpp:419:1:   required from here
../../thirdparty/gtest/include/gtest/gtest.h:1601:28: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
 GTEST_IMPL_CMP_HELPER_(GT, >);

````