Invalid remote sampling test in postgres_fdw.

Patch 0001 concerns what appears to be a bug in the postgres_fdw regression
tests.

As it was, the tests set the sampling method for the foreign server, but
rather than analyzing a table on that foreign server, the local table
analyze_table was analyzed instead.

The analyze commands now analyze a remote table, and the remote table
definition links it to the local table that was previously being analyzed
repeatedly.

Patch 0002 isn't a bug, just an annoyance.

It creates two psql variables which can then be interpolated into SQL
commands as string literals, eliminating the need to wrap commands in a DO
block. This makes the queries a bit more readable, and allows for any
errors (however unlikely) in those commands to be reported individually,
rather than grouped and potentially obfuscated by the DO block.