Fixed incorrect test in functional/routes/api/posts_spec.js
Fixed incorrect test in functional/routes/api/posts_spec.js

no issue

- `post.author_id` has no reference to any table currently, see https://github.com/TryGhost/Ghost/blob/1.21.3/core/server/data/schema/schema.js#L19
- that's why it is right now possible to insert none existent author id's
- with multiple authors, this get's protected (see https://github.com/TryGhost/Ghost/pull/9426)
  - you would get a proper error message
  - it is not allowed to insert invalid author id's
  - as soon as you do `include=author` you would receive an error
- fixed one test case where we inserted an invalid author id via the API

ErisDS/Ghost,kevinansfield/Ghost,Gargol/Ghost,janvt/Ghost,Kikobeats/Ghost,dbalders/Ghost,janvt/Ghost,bitjson/Ghost,JohnONolan/Ghost,e10/Ghost,bitjson/Ghost,e10/Ghost,Kikobeats/Ghost,TryGhost/Ghost,Kikobeats/Ghost,JohnONolan/Ghost,dbalders/Ghost,TryGhost/Ghost,ErisDS/Ghost,JohnONolan/Ghost,TryGhost/Ghost,acburdine/Ghost,kevinansfield/Ghost,ErisDS/Ghost,acburdine/Ghost,acburdine/Ghost,kevinansfield/Ghost,dbalders/Ghost,Gargol/Ghost,chevex/undoctrinate