samples: tests: Fix usb.audio.headphones_microphone test

The sample was failing twister test with a timeout because there was
no pass/fail criteria for it (nothing was tested). The fix adds
harness on consolse and some output that can be verified.

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>

samples/subsys/usb/audio/headphones_microphone/sample.yaml

@@ -5,3 +5,12 @@ tests:
     depends_on: usb_device
     tags: usb
     platform_allow: nrf52840dk_nrf52840 nrf5340dk_nrf5340_cpuapp
+    harness: console
+    harness_config:
+      type: multi_line
+      ordered: true
+      regex:
+        - "Entered main"
+        - "Found USB Headphones Device"
+        - "Found USB Microphone Device"
+        - "USB enabled"

samples/subsys/usb/audio/headphones_microphone/src/main.c

@@ -76,11 +76,15 @@ void main(void)
 		return;
 	}
 
+	LOG_INF("Found USB Headphones Device");
+
 	if (!mic_dev) {
 		LOG_ERR("Can not get USB Microphone Device");
 		return;
 	}
 
+	LOG_INF("Found USB Microphone Device");
+
 	usb_audio_register(hp_dev, &hp_ops);
 
 	usb_audio_register(mic_dev, &mic_ops);
@@ -90,4 +94,6 @@ void main(void)
 		LOG_ERR("Failed to enable USB");
 		return;
 	}
+
+	LOG_INF("USB enabled");
 }