_NestingGroup
   add usage attribute
   change 'fn' to 'testfn'
   container add_nest_group can take title
   group add_nested_group can take a group as argument
   group add_argument can take action as argument

change 'cross_tests' to 'usage_tests'

stack22929087.py - custom test can be added to group or registry
   example of a group with custom test and usage, but not arguments
change _NestingGroup to UsageGroup (and add to * list)
change nested_group to usage_group

The idea is that this type of group is used in the usage line, and that
the testing also focuses on usage.

It can also be subclassed like Action (need to test this)