[3.11] gh-106970: Fix Argument Clinic 'destination <name> clear' command (#106972) (#107059)

Add test for the 'destination <name> clear' command,
and the 'destination' directive in general.

Fix two bugs in 'destination <name> clear' command:

1. The text attribute of the allocator is called 'text', not '_text'
2. Return after processing the 'clear' command,
   instead of proceeding directly to the fail().

(cherry picked from commit 3372bcba98)

Tools/clinic/clinic.py

@@ -1798,7 +1798,7 @@ def __getitem__(self, i):
 
     def clear(self):
         for ta in self._array:
-            ta._text.clear()
+            ta.text.clear()
 
     def dump(self):
         texts = [ta.output() for ta in self._array]
@@ -4121,14 +4121,19 @@ def directive_set(self, name, value):
 
         self.clinic.__dict__[name] = value
 
-    def directive_destination(self, name, command, *args):
-        if command == 'new':
-            self.clinic.add_destination(name, *args)
-            return
-
-        if command == 'clear':
-            self.clinic.get_destination(name).clear()
-        fail("unknown destination command", repr(command))
+    def directive_destination(
+            self,
+            name: str,
+            command: str,
+            *args
+    ) -> None:
+        match command:
+            case "new":
+                self.clinic.add_destination(name, *args)
+            case "clear":
+                self.clinic.get_destination(name).clear()
+            case _:
+                fail("unknown destination command", repr(command))
 
 
     def directive_output(self, command_or_name, destination=''):