Adding tests for inheritance trees with temporary tables

Hi all,

While look at the handling of temporary relations with partition trees,
I have noticed that there are no tests for inheritance trees with temp
tables.  This has been mentioned here:
https://www.postgresql.org/message-id/20180618060200.GG3721@paquier.xyz

Attached is a patch to close the gap.  That's of course not v11
material, so I am parking this patch into the next CF.

Thanks,
--
Michael