Change `ReactMultiChild` test to check for `innerHTML` descriptor.

Not all testing environments will support setting the `innerHTML` descriptor. For example, PhantomJS initializes the `innerHTML` property as not configurable.
Fix `Danger` test failures.

The original tests were flawed because the `Danger` module exploits the fact that all React-generated markup has at least one attribute. This allows the module to extract node names from markup strings faster.

However, the tests were passing in strings of markup with no attributes.

Also, this fixes a test failure due to the test trying to set text content into a `<tr>` which is typically disallowed by browsers (and PhantomJS). This changes it to use `<td>` instead.