Add behavioral test coverage for DA-native restore payload + hooks

alt_mysql_restore_payload.sh (the DirectAdmin native-restore hook that
imports a plugin-origin backup payload into alt-mariadb) had zero
behavioral test coverage - only hook-installation plumbing was tested.
Add a fake mariadb/mariadb-dump client stub (tracking a simple
existing-databases state file) to exercise the real script end-to-end
without a live server, covering: checksum mismatch rejection,
overwrite=deny enforcement against an existing database, and the
happy-path restore. Add a DA_ALT_MYSQL_SETTINGS_FILE override (same
pattern as worker.sh's PLUGIN_DIR) so the script's settings file can be
pointed at a test fixture instead of the real system one.

Also extend da_integration_install_test.sh to check hook-symlink
install/uninstall coverage for database_delete_pre.sh,
database_user_password_change_pre.sh, database_user_create_post.sh,
and database_destroy_user_post.sh, which were previously untested
(only database_create_pre.sh was checked).

scripts/da-integration/alt_mysql_restore_payload.sh

@@ -4,7 +4,7 @@ set -Eeuo pipefail
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 PLUGIN_DIR="$(cd "$SCRIPT_DIR/../.." && pwd)"
 COMMON_FILE="$PLUGIN_DIR/scripts/setup/mysql_common.sh"
-SETTINGS_FILE="$PLUGIN_DIR/plugin-settings.conf"
+SETTINGS_FILE="${DA_ALT_MYSQL_SETTINGS_FILE:-$PLUGIN_DIR/plugin-settings.conf}"
 LOG_FILE="${DA_ALT_MYSQL_RESTORE_LOG:-$PLUGIN_DIR/data/logs/da-restore.log}"
 DA_USER=""
 PAYLOAD_DIR=""