In some external test suites, what is defined as a "test" can not be
run by itself and need a specific runner script/tool.

This introduces the concept of an "inner runner", that is, a custom
test runner or tool that can deal with custom tests that do follow
the Avocado SIMPLE test definition.

This implements the Trello card:

 https://trello.com/c/TpXecE2n/486-introduce-inner-runner-to-avocado-run

More information can be found in the man page section about it.

Changes from v1:
 * removed "level" from "inner level runner" mentions
 * replicated docs from man page to "main" docs
Signed-off-by: NCleber Rosa <crosa@redhat.com>