pgsql: pg_stat_statements: Add some tests for older versions still usab

pg_stat_statements: Add some tests for older versions still usable

When the newest version is loaded, the backend would load objects from
the oldest complete SQL file (here 1.4) and then update to the latest
version with transition scripts (up to 1.9 currently). This provides
some coverage for upgrades of pg_stat_statements, but there is no test
to show how things have changed across each version.

This adds a couple of tests for the upgrade paths using objects from
each version supported, stressing the objects whose behaviors have
changed across each version supported.

Author: Erica Zhang
Reviewed-by: Julien Rouhaud, Michael Paquier
Discussion: https://postgr.es/m/tencent_BBA974AFF61379F2345E782FD6C55891950A@qq.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/2b0da0365bec6c62cc9c5c317bab6cbee3d52ef4

Modified Files
--------------
contrib/pg_stat_statements/Makefile | 2 +-
.../pg_stat_statements/expected/oldextversions.out | 139 +++++++++++++++++++++
contrib/pg_stat_statements/sql/oldextversions.sql | 39 ++++++
3 files changed, 179 insertions(+), 1 deletion(-)