test: fix signed/unsigned compare warning.

test: fix signed/unsigned compare warning.

 In file included from test/objectstore/chain_xattr.cc:29:0:
../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 = long unsigned int; T2 =
int]’:
../src/gmock/gtest/include/gtest/gtest.h:1484:23:   required from
‘static testing::AssertionResult
testing::internal::EqHelper<lhs_is_null_literal>::Compare(const char*,
const char*, const T1&, const T2&) [with T1 = long unsigned int; T2 =
int; bool lhs_is_null_literal = false]’
test/objectstore/chain_xattr.cc:205:314:   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: Jianpeng Ma <jianpeng.ma@intel.com>