[improv_serial] Add uart bus support and host integration test (#18794)

tests/components/improv_serial/common-uart-bus.yaml

@@ -0,0 +1,11 @@
+wifi:
+  ssid: MySSID
+  password: password1
+
+# Serial logging off; on a dedicated UART bus improv_serial must not
+# require the logger's serial settings
+logger:
+  baud_rate: 0
+
+improv_serial:
+  uart_id: uart_bus

tests/components/improv_serial/test-uart-bus.esp32-idf.yaml

@@ -0,0 +1,3 @@
+packages:
+  uart: !include ../../test_build_components/common/uart/esp32-idf.yaml
+  improv_serial: !include common-uart-bus.yaml

tests/components/improv_serial/test-uart-bus.esp8266-ard.yaml

@@ -0,0 +1,3 @@
+packages:
+  uart: !include ../../test_build_components/common/uart/esp8266-ard.yaml
+  improv_serial: !include common-uart-bus.yaml