[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 3372bcba9893030e4063a9264ec0b4d1b6166883)