Help: Document add_custom_command marking outputs GENERATED
Tests: Test add_custom_command and add_custom_target error cases

Add RunCMake.add_custom_command and RunCMake.add_custom_target tests to
cover the error messages for these commands.
Merge topic 'doc-add_custom_command' into next

0cacf739 Help: Document add_custom_command marking outputs GENERATED
Merge topic 'test-custom-command-errors' into next

758386d8 Tests: Test add_custom_command and add_custom_target error cases