Allow a plain function to be used as an assertion

Allow a plain function to be used as an assertion: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). Right now the only things you can pass as the second argument to expect are an assertion string ( expect(2, 'to be a number') ) or an expect.it(...) expression. If…