fix(typings): remove union type of testAction

[Brooooooklyn]

没有必要写复杂的联合类型，会带来额外的麻烦。
比如:

// typeError, No best common type exists among yield expressions.
it('test should pass', function *() {
   yield Promise.resolve(1)
   yield Observable.of(1)
})

直接自动推断为 any 就好了

[coveralls]

Coverage remained the same at 91.255% when pulling 7a93feb on Brooooooklyn:issue/typings into 30e06c8 on thunks:master.

[Brooooooklyn]

@zensh