fix `rasa test` when one token has multiple entity annotations

[ka-bu]

Rasa Open Source version

2.8

Rasa SDK version

No response

Rasa X version

No response

Python version

3.8

What operating system are you using?

OSX

What happened?

more context and initial solutions in #10394 (2.8.x) and #10404 (#3.0.x)

Command / Request

No response

Relevant log output

No response

Definition of Done:

• [x] The proposed solution investigated in terms of data format, generalizability across customers.
• [x] Get other teams (CSE, Enable) aligned on the proposed solution
• [x] #10780
  • [x] Review the PR
  • [x] New version with enhancement released
  • [x] Decide if the fix should be backported to 2.8.x
  • [x] Docs are updated

[JEM-Mosig]

We decided together with CSE that the proposed format is fine.

[JEM-Mosig]

PR merged into 2.8.x