WIP Add the rest of the tests for what we expect re: unknown attributes

**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

src/renderers/dom/shared/__tests__/ReactDOMAttribute-test.js

@@ -51,5 +51,59 @@ describe('ReactDOMAttribute', () => {
       ReactDOM.render(<div />, el);
       expect(el.firstChild.hasAttribute('unknown')).toBe(false);
     });
+
+    it('coerces unknown attributes to strings with numbers and booleans', () => {
+      var el = document.createElement('div');
+
+      function testCoerceToString(value) {
+        ReactDOM.render(<div unknown="something" />, el);
+        expect(el.firstChild.getAttribute('unknown')).toBe('something');
+        ReactDOM.render(<div unknown={value} />, el);
+        expect(el.firstChild.getAttribute('unknown')).toBe(value + '');
+      }
+
+      testCoerceToString(0);
+      testCoerceToString(-1);
+      testCoerceToString(42);
+      testCoerceToString(9000.99999);
+      // TODO: either change what we expect here or update the implementation
+      // so that these pass -
+      //
+      // testCoerceToString(true);
+      // testCoerceToString(false);
+    });
+
+    // TODO: get this test passing
+    xit('coerces unknown attributes to strings **and warns** with NaN, symbols, functions, and objects', () => {
+      var el = document.createElement('div');
+      spyOn(console, 'error');
+
+      function testCoerceToString(value) {
+        ReactDOM.render(<div unknown="something" />, el);
+        expect(el.firstChild.getAttribute('unknown')).toBe('something');
+        expectDev(console.error.calls.count(0)).toBe(0);
+        ReactDOM.render(<div unknown={value} />, el);
+        expect(el.firstChild.getAttribute('unknown')).toBe(value + '');
+        expectDev(console.error.calls.count(0)).toBe(1);
+        // TODO: add specific expectations about what the warning says
+        // expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(...
+        console.error.calls.reset();
+      }
+
+      // TODO: this does not warn. We think it should.
+      testCoerceToString(NaN);
+
+      // TODO: either change what we expect or change our implementation
+      // this throws "TypeError: Cannot convert a Symbol value to a string"
+      // testCoerceToString(Symbol('foo'));
+
+
+      // TODO: either change what we expect or change our implementation
+      // this does not set it to the stringified function.
+      testCoerceToString(() => 'foo');
+
+      // TODO: this does not warn. We think it should.
+      testCoerceToString({hello: 'world'});
+    });
   });
 });