Start a separate test suite for plpgsql

Start a separate test suite for plpgsql

The plpgsql.sql test file in the main regression tests is now by far the
largest after numeric_big, making editing and managing the test cases
very cumbersome.  The other PLs have their own test suites split up into
smaller files by topic.  It would be nice to have that for plpgsql as
well.  So, to get that started, set up test infrastructure in
src/pl/plpgsql/src/ and split out the recently added procedure test
cases into a new file there.  That file now mirrors the test cases added
to the other PLs, making managing those matching tests a bit easier too.

msvc build system changes with help from Michael Paquier