Add more unit testing examples to guide

[chrisvfritz]

These aren't really Vue-specific testing tips, but I think they could still be helpful for people. This is a list that will probably grow as I (or others) think of more things.

• Using spies to ensure something happens in a lifecycle hook
• Mocking Ajax requests
• Mocking external dependencies

Vuex stores/modules

• Testing initial state
• Testing actions
• Testing mutations
• Testing getters

[LinusBorg]

Don't the vuex-related example belong into the vuex docs?

[chrisvfritz]

Yeah, that's a good idea. I'd like to link to it though.

[adiakritos]

+10,000 for Mocking external dependencies there is 1 example and it's so brief I'm left to experimenting and searching for guides to really get it.

[sdras]

What do we think of this now that we have linked up vue-test-utils and have made a cookbook recipe? Does that close out this issue?

#1470

[sdras]

Gonna close this out since we now point to vue-test-utils, but feel free to reopen