pgsql: 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(at)kurilemu(dot)de>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Man Zeng <zengman(at)halodbtech(dot)com>
Discussion: https://postgr.es/m/aW6_Xc8auuu5iAPi@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f9afd56218af0d704c12540a1b5e893d196f6aa7

Modified Files
--------------
contrib/pg_stat_statements/Makefile | 22 +++++++++++++++++++---
contrib/pg_stat_statements/expected/squashing.out | 1 -
contrib/pg_stat_statements/meson.build | 3 ++-
contrib/pg_stat_statements/sql/squashing.sql | 1 -
4 files changed, 21 insertions(+), 6 deletions(-)