fix: adjust UT_ASSERT to gcc 7.5.0 from openSUSE Leap

It fixes the following compilation error:

In file included from /libpmemobj-cpp/tests/concurrent_hash_map_tx/concurrent_hash_map_tx.cpp:38:0:
/libpmemobj-cpp/tests/concurrent_hash_map_tx/concurrent_hash_map_tx.cpp: In function 'void test_tx_singlethread(pmem::obj::pool<root>&)':
/libpmemobj-cpp/tests/concurrent_hash_map_tx/concurrent_hash_map_tx.cpp:359:27: error: conversion to 'pmem::obj::concurrent_hash_map<pmem::obj::p<int>, pmem::obj::p<int> >::size_type {aka long unsigned int}' from 'int' may change the sign of the result [-Werror=sign-conversion]
  UT_ASSERTeq(map->size(), static_cast<size_t>(number_of_inserts));
/libpmemobj-cpp/tests/common/unittest.hpp:151:21: note: in definition of macro 'UT_ASSERTeq'
  ((void)(((lhs) == (rhs)) ||                                            \
                     ^~~