Add Common Test Sub Group example

[ansd]

With this type of specification execution of unwanted groups (in otherwise matching paths), and/or the execution of subgroups can be avoided. The command line syntax of the group path is a list of group names in the path

https://www.erlang.org/doc/apps/common_test/run_test_chapter.html#test-case-group-execution

[essen]

Merged, thanks!