test: silence warning from -Wsign-compare

test: silence warning from -Wsign-compare

The following warning appears during make:
In file included from ceph/src/test/librados/TestCase.h:9:0,
                 from ceph/src/test/librados/c_read_operations.cc:10:
ceph/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]’:
ceph/src/googletest/googletest/include/gtest/gtest.h:1421:23:   required from ‘static testing::AssertionResult testing::internal::EqHelper<lhs_is_null_literal>::Compare(const char*, const char*, const T1&, const T2&) [with T1 = int; T2 = long unsigned int; bool lhs_is_null_literal = false]’
ceph/src/test/librados/c_read_operations.cc:685:3:   required from here
ceph/src/googletest/googletest/include/gtest/gtest.h:1392:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (lhs == rhs) {
       ~~~~^~~~~~

Signed-off-by: Jos Collin <jcollin@redhat.com>