pgsql-server: Change some of the existing plpgsql regression test cases

Log Message:
-----------
Change some of the existing plpgsql regression test cases so that they
exercise dollar quoting and named function parameters. AFAICS we had
no tests of either feature before.

Modified Files:
--------------
pgsql-server/src/test/regress/expected:
plpgsql.out (r1.20 -> r1.21)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/test/regress/expected/plpgsql.out.diff?r1=1.20&r2=1.21)
pgsql-server/src/test/regress/sql:
plpgsql.sql (r1.15 -> r1.16)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/test/regress/sql/plpgsql.sql.diff?r1=1.15&r2=1.16)