test/objectstore: fix a -Wsign-compare warning in compiling.

test/objectstore: fix a -Wsign-compare warning in compiling.

In file included from test/objectstore/store_test.cc:33:0:
../src/gmock/gtest/include/gtest/gtest.h: In instantiation of ‘testing::AssertionResult testing::internal::CmpHelperEQ(const char*, const char*, const T1&, const T2&) [with T1 = unsigned int; T2 = int]’:
../src/gmock/gtest/include/gtest/gtest.h:1524:23:   required from ‘static testing::AssertionResult testing::internal::EqHelper<true>::Compare(const char*, const char*, const T1&, const T2&, typename testing::internal::EnableIf<(! testing::internal::is_pointer<T2>::value)>::type*) [with T1 = unsigned int; T2 = int; typename testing::internal::EnableIf<(! testing::internal::is_pointer<T2>::value)>::type = void]’
test/objectstore/store_test.cc:1088:285:   required from here
../src/gmock/gtest/include/gtest/gtest.h:1448:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (expected == actual) {
                ^

Signed-off-by: Dongsheng Yang <dongsheng.yang@easystack.cn>