text: Title: Add `full_name` to replace `longname` to suite and test objects
  Body: Both `TestSuite` and `TestCase` objects have a `longname` attribute that contains the name of the suite/test prefixed with the `longname` of the parent suite. The functionality is useful, but the name doesn't follow our coding style and is bad in general. We are enhancing our APIs also otherwise and the result side `Keyword` object just got `full_name` for similar purpose than `longname` (#4884). That name works well also with suites and tests.
  
  The plan is to do the following:
  1. In RF 7.0 introduce the `full_name` property.
  2. Preserve `longname` in RF 7.0. Mention that it is deprecated in its documentation but don't emit actual deprecation warnings.
  3. Deprecate `longname` "loudly" in RF 8.0 or possibly later.
  4. In some suitable future release remove `longname`.
  
  This issue covers the two first steps above.
  
  
  