Add option to disable for tests

I have a setup where I'm using Jest and vue-test-utils to mount and test my components. Some of the inner components use the v-observe-visibility directive. However, it breaks/slows down considerably the tests. It looks like it really doesn't like being run in testing virtual DOM. I got around this by adding:

`Vue.prototype.isTest = process.env.JEST_WORKER_ID !== undefined`

in my startup config. And then a

`v-observe-visibility="isTest ? false : onRulesVisible"` i

nside my components. However, this feels very clunky. It'd be nice if there were a hook we could access to disable the directive when running tests. I'll admit, I haven't taken a look at the inner workings of the directive. I can't be the only one who has run into this problem.