os/tests: silence -Wsign-compare warning

os/tests: silence -Wsign-compare warning

silence warning like

In file included from
/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/test/objectstore/store_test.cc:25:0:
/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/googletest/googletest/include/gtest/gtest.h:
In instantiation of 'testing::AssertionResult
testing::internal::CmpHelperEQ(const char*, const char*, const T1&,
const T2&) [with T1 = int; T2 = long unsigned int]':

Signed-off-by: Kefu Chai <kchai@redhat.com>