Actually check results in test_serialport

Previously, tests were reported successful in every case. For example
even with non-existent devices. So make the serial ports to use for
testing explicit and actually check for success.

.gitignore

@@ -2,3 +2,4 @@ target
 Cargo.lock
 *.bk
 *~
+.env

tests/test_serialport.rs

@@ -4,6 +4,9 @@
 use serialport::*;
 use std::time::Duration;
 
+const PORT_1: &str = env!("SERIALPORT_RS_TEST_PORT_1");
+const PORT_2: &str = env!("SERIALPORT_RS_TEST_PORT_2");
+
 #[test]
 fn test_listing_ports() {
     let ports = serialport::available_ports().expect("No ports found!");
@@ -14,44 +17,46 @@ fn test_listing_ports() {
 
 #[test]
 fn test_opening_found_ports() {
+    // There is no guarantee that we even might open the ports returned by `available_ports`. But
+    // the ports we are using for testing shall be among them.
     let ports = serialport::available_ports().unwrap();
-    for p in ports {
-        let _port = serialport::new(p.port_name, 9600).open();
-    }
+    assert!(ports.iter().any(|info| info.port_name == PORT_1));
+    assert!(ports.iter().any(|info| info.port_name == PORT_2));
 }
 
 #[test]
 fn test_opening_port() {
-    let _port = serialport::new("/dev/ttyUSB0", 9600).open();
+    serialport::new(PORT_1, 9600).open().unwrap();
 }
 
 #[test]
 fn test_opening_native_port() {
-    let _port = serialport::new("/dev/ttyUSB0", 9600).open_native();
+    serialport::new(PORT_1, 9600).open_native().unwrap();
 }
 
 #[test]
 fn test_configuring_ports() {
-    let _port = serialport::new("/dev/ttyUSB0", 9600)
+    serialport::new(PORT_1, 9600)
         .data_bits(DataBits::Five)
         .flow_control(FlowControl::None)
         .parity(Parity::None)
         .stop_bits(StopBits::One)
         .timeout(Duration::from_millis(1))
-        .open();
+        .open()
+        .unwrap();
 }
 
 #[test]
 fn test_duplicating_port_config() {
-    let port1_config = serialport::new("/dev/ttyUSB0", 9600)
+    let port1_config = serialport::new(PORT_1, 9600)
         .data_bits(DataBits::Five)
         .flow_control(FlowControl::None)
         .parity(Parity::None)
         .stop_bits(StopBits::One)
         .timeout(Duration::from_millis(1));
 
-    let port2_config = port1_config.clone().path("/dev/ttyUSB1").baud_rate(115_200);
+    let port2_config = port1_config.clone().path(PORT_2).baud_rate(115_200);
 
-    let _port1 = port1_config.open();
-    let _port1 = port2_config.open();
+    let _port1 = port1_config.open().unwrap();
+    let _port1 = port2_config.open().unwrap();
 }