Switch TAP tests of pg_rewind to use role with only functionpermissions

Hi all,

Recent commit bfc80683 has added some documentation in pg_rewind about
the fact that it is possible to do the operation with a non-superuser,
assuming that this role has sufficient grant rights to execute the
functions used by pg_rewind.

Peter Eisentraut has suggested to have some tests for this kind of
user here:
https://www.postgresql.org/message-id/e1570ba6-4459-d9b2-1321-9449adaaef4c@2ndquadrant.com

Attached is a patch which switches all the TAP tests of pg_rewind to
do that.  As of now, the tests depend on a superuser for everything,
and it seems to me that it makes little sense to make the tests more
pluggable by being able to switch the roles used on-the-fly (the
invocation of pg_rewind is stuck into RewindTest.pm) as a superuser
has no restrictions.

Any thoughts?
--
Michael