增加了User的一个Mock，并写了管理员登录的测试 by lhw
Merge branch 'master' of ssh://git@github.com/minusone13/InvoicingSystem.git
Merge branch 'master' of ssh://git@github.com/minusone13/InvoicingSystem.git
Merge branch 'master' of ssh://git@github.com/minusone13/InvoicingSystem.git