pg_stat_statements: Expand tests for SET statements

pg_stat_statements: Expand tests for SET statements

There are many grammar flavors that depend on the parse node
VariableSetStmt.  This closes the gap in pg_stat_statements by providing
test coverage for what should be a large majority of them, improving more
the work begun in de2aca288569.  This will be used to ease the
evaluation of a path towards more normalization of SET queries with
query jumbling.

Note that SET NAMES (grammar from the standard, synonym of SET
client_encoding) is omitted on purpose, this could use UTF8 with a
conditional script where UTF8 is supported, but that does not seem worth
the maintenance cost for the sake of these tests.

The author has submitted most of these in a TAP test (filled in any
holes I could spot), still queries in a SQL file of pg_stat_statements
is able to achieve the same goal while being easier to look at when
testing normalization patterns.

Author: Greg Sabino Mullane, Michael Paquier
Discussion: https://postgr.es/m/CAKAnmmJtJY2jzQN91=2QAD2eAJAA-Per61eyO48-TyxEg-q0Rg@mail.gmail.com