Update FastAPI example test to use AnyIO

[ljhenne]

Description

• Use client.portal.call rather than client.get_event_loop().run_until_complete to run async function get_user_by_db().
• Replace event_loop fixture with portal fixture which just ensures that the portal has been instantiated within the client so that the Optional can be dropped from the type hint.

Motivation and Context

Fixes #1029. See comment #1029 (comment) for details.

How Has This Been Tested?

I ran make test and ensured all tests passed.

Checklist:

• My code follows the code style of this project.

• I have added the changelog accordingly.

• I have read the CONTRIBUTING document.

• All new and existing tests passed.
  - [ ] My change requires a change to the documentation.
  - [ ] I have updated the documentation accordingly.
  - [ ] I have added tests to cover my changes.

• Did not add tests since this was to fix an existing test.

• It looks like documentation should automatically pull the updated example code.