[water_heater] (4/4) Implement tests for new water_heater component (#12517)

Co-authored-by: pre-commit-ci-lite[bot] <117423508+pre-commit-ci-lite[bot]@users.noreply.github.com>
Co-authored-by: J. Nick Koston <nick@koston.org>
Co-authored-by: J. Nick Koston <nick@home-assistant.io>

tests/components/water_heater/common.yaml

@@ -0,0 +1,16 @@
+water_heater:
+  - platform: template
+    id: my_boiler
+    name: "Test Boiler"
+    min_temperature: 10
+    max_temperature: 85
+    target_temperature_step: 0.5
+    current_temperature_step: 0.1
+    optimistic: true
+    current_temperature: 45.0
+    mode: !lambda "return water_heater::WATER_HEATER_MODE_ECO;"
+    visual:
+      min_temperature: 10
+      max_temperature: 85
+      target_temperature_step: 0.5
+      current_temperature_step: 0.1

tests/components/web_server/common.yaml

@@ -36,3 +36,4 @@ datetime:
     optimistic: yes
 event:
 update:
+water_heater: