Actor unitest example

[m3nax]

Description

Added to the Actor example some UnitTests to demonstrate how to write them.

Issue reference

• 1230

• 1305

Please reference the issue this PR will close: #[issue number]

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• [x] Code compiles correctly
• [x] Created/updated tests
• [ ] Extended the documentation

[m3nax]

@philliphoff please remove this pull from 1.15 milestone, I have prepared a new pull #1326 that includes the example and the correction for the signature of actor methods.