Add tests for loading the catalog

tests/pgintegration/DbTest.h

@@ -17,6 +17,8 @@ protected:
     Pgsql::Connection& conn() { return *conn_; }
     QString dbName() const { return dbName_; }
     QString dbUrl() const;
+    /// Opens a new connection to this test's database as the plain (non-superuser) role.
+    std::unique_ptr<Pgsql::Connection> plainConn() const;
 
 private:
     QString dbName_;