Switch on/off all lights, and wait for the result (#27078)

* Switch on/off all lights, and wait for the result

Reuses the parallel_updates semaphore.
This is a small crutch which serializes platforms which already do tis
for updates. Platforms which can parallelize everything, this makes it
go faster

* Fix broken unittest

With manual validation, with help from @frenck, we found out that the
assertions are wrong and the test should be failing.

The sequence requested is
OFF
ON

without cancelation, this code should result in:
off,off,off,on,on,on

testable, by adding a `await hass.async_block_till_done()` between the
off and on call.

with cancelation. there should be less off call's so
off,on,on,on

* Adding tests for async_request_call

* Process review feedback

* Switch gather with wait

* 👕 running black

homeassistant/helpers/entity.py

@@ -551,6 +551,19 @@ class Entity:
         """Return the representation."""
         return "<Entity {}: {}>".format(self.name, self.state)
 
+    # call an requests
+    async def async_request_call(self, coro):
+        """Process request batched."""
+
+        if self.parallel_updates:
+            await self.parallel_updates.acquire()
+
+        try:
+            await coro
+        finally:
+            if self.parallel_updates:
+                self.parallel_updates.release()
+
 
 class ToggleEntity(Entity):
     """An abstract class for entities that can be turned on and off."""