pg_stat_statements: Rework test order

pg_stat_statements: Rework test order

The test "squashing" was the last item of the REGRESS list, but
"cleanup" should be the second to last, dropping the extension.
"oldextversions" is the last item.

In passing, the REGRESS list is cleaned up to include one item per line,
so as diffs are minimized when adding new test files.

Noticed while playing with this area of the code.

Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Man Zeng <zengman@halodbtech.com>
Discussion: https://postgr.es/m/aW6_Xc8auuu5iAPi@paquier.xyz