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

[erlend-aasland]

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)

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

[erlend-aasland]

FTR, test_unknown_destination_command() was accidentally added in gh-107002. Originally it was a part of gh-106972, so it should have been a part of this backport as well.