Silence compiler warnings
Silence compiler warnings

test/pi_table.cpp:60:16: error:
      comparison of integers of different signs: 'uint64_t'
      (aka 'unsigned long') and 'int64_t' (aka 'long')
[-Werror,-Wsign-compare]
  while (prime < pi.size())
         ~~~~~ ^ ~~~~~~~~~
/home/travis/build/kimwalisch/primecount/test/pi_table.cpp:63:21:
error:
      comparison of integers of different signs: 'int64_t' (aka 'long')
and
      'uint64_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
    check(pi[prime] == count);
          ~~~~~~~~~ ^  ~~~~~
/home/travis/build/kimwalisch/primecount/test/pi_table.cpp:72:17:
error:
      comparison of integers of different signs: 'int64_t' (aka 'long')
and
      'uint64_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
    check(pi[n] == primesieve::count_primes(0, n));

kimwalisch/primecount,kimwalisch/primecount,kimwalisch/primecount