Add tests for inheritance trees mixing permanent and temporary relations

Add tests for inheritance trees mixing permanent and temporary relations

While working on 1c7c317c and related things, which has clarified the
use of partitions with temporary tables, I have noticed that there could
be better coverage for inheritance trees mixing temporary and permanent
relations.  A lot of cross-checks happen in MergeAttributes() which is
not designed for this purpose, so the tests added in this commit will
make sure that any kind of future refactoring will limit the amount of
compatibility breakage.

Author: Michael Paquier
Reviewed-by: Ashutosh Bapat
Discussion: https://postgr.es/m/20180619022131.GE3314@paquier.xyz