[3.12] gh-106970: Fix Argument Clinic 'destination <name> clear' command (GH-106972) (#106983)

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 3372bcba9893030e4063a9264ec0b4d1b6166883)

Co-authored-by: Erlend E. Aasland <erlend@python.org>