Several statements need to reference the current connection's current
database name and current port value.  Until now, this has been
accomplished by creating dynamic SQL statements inside of a DO block,
which is not as easy to parse.  It also takes away some of the
granularity of any error messages that might occur, making debugging
harder.

By capturing the connection-specific settings into psql variables, it
becomes possible to write simpler SQL statements for the FDW objects.
This eliminates most of DO blocks used in this test, making it a bit
more readable and shorter.

Author: Author: Corey Huinker <corey.huinker@gmail.com>
Discussion: https://postgr.es/m/CADkLM=cpUiJ3QF7aUthTvaVMmgQcm7QqZBRMDLhBRTR+gJX-Og@mail.gmail.com