Since these names are global, using the same ones in different regression
tests creates a hazard of test failures if any two such scripts run
concurrently.  Let's establish a policy of not doing that.  In the cases
where a conflict existed, I chose to rename both sides: in principle one
script or the other could've been left in possession of the common name,
but that seems to just invite more trouble of the same sort.

There are a number of places where scripts are using names that seem
unduly generic, but in the absence of actual conflicts I left them alone.

In addition, fix insert.sql's use of "someone_else" as a role name.
That's a flat out violation of longstanding project policy, so back-patch
that change to v10 where the usage appeared.  The rest of this is just
future-proofing, as no two of these scripts are actually run concurrently
in the existing parallel_schedule.

Conflicts of schema-qualified names also exist, but will be dealt with
separately.

Discussion: https://postgr.es/m/4627.1521070268@sss.pgh.pa.us