Improve test db management (#747)

test/fixtures/database.sql

@@ -1,6 +1,3 @@
-DROP DATABASE IF EXISTS postgrest_test;
-DROP ROLE IF EXISTS postgrest_test;
-CREATE USER postgrest_test createdb createrole;
-CREATE DATABASE postgrest_test OWNER postgrest_test;
+CREATE EXTENSION IF NOT EXISTS pgcrypto;
 
-ALTER DATABASE postgrest_test SET request.jwt.claim.id = '-1';
+ALTER DATABASE :db SET request.jwt.claim.id = '-1';