**what is the change?:**
Adds tests for the following behavior -

- Numbers and booleans should be converted to strings, and not warn
- NaN, Symbols, functions, and objects should be converted to strings,
  and *should* warn

Going to add tests for the not-warning behavior in a follow-up.

These tests are not entirely passing - we either need to change what we
expect or change the behavior.

**why make this change?:**
Gets everyone on the same page about expected behavior, and codifies it
in a maintainable way

**test plan:**
`yarn test src/renderers/dom/shared/__tests__/ReactDOMAttribute-test.js`

**issue:**
facebook#10399

**what is the change?:**
We are testing the behavior of unknown attributes, which has changed
since React 15.

We want to *not* warn for the following cases -
- null
- undefined
- missing
- strings
- numbers
- booleans

**why make this change?:**
We want to verify that warnings don't get fired at the wrong time.

**test plan:**
`yarn test src/renderers/dom/shared/__tests__/ReactDOMAttribute-test.js`

**issue:**
facebook#10399

…warn

I don't think we use this convention anywhere.

**what is the change?:**
- booleans don't get stringified
- some warnings have changed since we originally wrote this

**why make this change?:**
The attribute behavior is finalized and now we can test it :D

I also found it handy to have a row with a truly unknown attribute, so
added "imaginaryFriend".

**test plan:**
`yarn test src/renderers/dom/shared/__tests__/ReactDOMAttribute-test.js`
and comparing the tests to the attribute table

**issue:**
facebook#10399