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

[miss-islington]

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

Fix two bugs in 'destination 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 3372bcb)

Co-authored-by: Erlend E. Aasland [email protected]

• Issue: Argument Clinic 'destination <name> clear' is broken #106970