unit_tests.sh fails in last check with timeouted server connection

[ghorwin]

Running unit_tests.sh gives an assert in the unit-test-client.log:

...
7/8 Disable byte timeout: OK
8/8 Connection timeout: 
Line 679: assertion error for 'rc == -1 && (*__errno_location ()) == 110': 

This causes the test suite to fail.

[ghorwin]

Regression was introduced in 6ae4a76

   // 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);     // <-- BUG: should be invalid_ctx, right?
    rc = modbus_connect(invalid_ctx);
    printf("8/8 Connection timeout: ");
    ASSERT_TRUE(rc == -1 && errno == ETIMEDOUT, "");
    modbus_free(invalid_ctx);

When adjusting the response timeout in ctx rather the intendet invalid_ctx the connection fails with errno 111 (connection refused).

I'm not quite clear why this happens, as we try to use invalid_ctx in the connect statement and not ctx. The errno is set to ETIMEDOUT in modbus-tcp.c:310 correctly, when using the timeout on invalid_ctx. However, when we do not set the timeout, then the errno is set to ECONNREFUSED in modbus-tcp.c:319.

By changing ctx to invalid_ctx in the test code (see snipped above) the test suite runs correctly again. -> pull request