test: fix tests after minitest upgrade

Fixes fallout from e01bf7f. One `must_be_like` was missed.

Author: dr-itz

activerecord/test/cases/arel/select_manager_test.rb

@@ -377,7 +377,7 @@ def test_join_sources
         mgr.from table
         mgr.orders << Arel::Nodes::Ascending.new(Arel.sql("foo"))
         mgr.ast.grep(Arel::Nodes::OuterJoin)
-        mgr.to_sql.must_be_like %{ SELECT * FROM "users" ORDER BY foo ASC }
+        _(mgr.to_sql).must_be_like %{ SELECT * FROM "users" ORDER BY foo ASC }
       end
     end