inet_net_test: Use int to hold expected return values from inet_net_pton

GCC warns about the sign mismatch in comparisons:

lib/libc/tests/net/inet_net_test.cc: In member function 'virtual void {anonymous}::atfu_tc_inet_net_inet4::body() const':
lib/libc/tests/net/inet_net_test.cc:86:17: error: comparison of integer expressions of different signedness: 'int' and 'const unsigned int' [-Werror=sign-compare]

86 |                 ATF_REQUIRE_EQ(bits, addr.bits);
   |                 ^~~~~~~~~~~~~~

lib/libc/tests/net/inet_net_test.cc: In member function 'virtual void {anonymous}::atfu_tc_inet_net_inet6::body() const':
lib/libc/tests/net/inet_net_test.cc:205:17: error: comparison of integer expressions of different signedness: 'int' and 'const unsigned int' [-Werror=sign-compare]

205 |                 ATF_REQUIRE_EQ(bits, addr.bits);
    |                 ^~~~~~~~~~~~~~

Fixes: 8f4a0d2f7b96 ("libc: Import OpenBSD's inet_net_{ntop,pton}")