Add Amazon Linux 2 support, prepare for PG18

* Add Amazon Linux 2 as a supported OS.

* Dynamically select the package manager for Amazon and Oracle Linux.

* Prepare for supporting PostgreSQL 18.  Specifically, the Docker image
  seems to have moved the data directory, so get that dynamically.

tests/docker-compose.yml

@@ -20,7 +20,7 @@ services:
       POSTGRES_PASSWORD: secret
     healthcheck:
       #test: [ "CMD", "pg_isready", "-U", "postgres" ]
-      test: [ "CMD-SHELL", "pg_controldata /var/lib/postgresql/data/ | grep -q 'in production'" ]
+      test: [ "CMD-SHELL", "pg_controldata $(psql -At -U postgres -c 'show data_directory') | grep -q 'in production'" ]
       interval: 5s
       timeout: 2s
       retries: 40

tests/run-tests.sh

@@ -24,6 +24,7 @@ images["14"]='14-bookworm'
 images["15"]='15-bookworm'
 images["16"]='16-bookworm'
 images["17"]='17-bookworm'
+images["18"]='18-trixie'
 
 declare -A results=()