unit-test-client: The test fails in environments where networking is not available

[artyom-poptsov]

The change that was introduced in 6ae4a76 leads to test errors in environments where networking is not available (e.g. in GNU Guix build environment):

    // Invalid in TCP or RTU mode...
    modbus_t *invalid_ctx = modbus_new_tcp("1.2.3.4", 1502);
    modbus_set_response_timeout(ctx, 0, 1);
    rc = modbus_connect(invalid_ctx);
    printf("8/8 Connection timeout: ");
    ASSERT_TRUE(rc == -1 && errno == ETIMEDOUT, "");
    modbus_free(invalid_ctx);

Assertion fails due to errno being equal to 101 (ENETUNREACH) instead of 110 (ETIMEDOUT)

Currently I solved this problem by removing the code I mentioned above with a patch, but maybe we should add an option that disables such tests?