Styled system wrap-up, test fixes

[shawnbot]

This removes the last vestiges of our old class name mapping system in src/props.js and updates some of our tests to silence console errors and bring coverage back to nearly 100%.

[shawnbot]

As of 62f9356, coverage looks like this:

---------------------|----------|----------|----------|----------|-------------------|
File                 |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
---------------------|----------|----------|----------|----------|-------------------|
All files            |      100 |    98.26 |      100 |      100 |                   |
 Avatar.js           |      100 |      100 |      100 |      100 |                   |
 Block.js            |      100 |      100 |      100 |      100 |                   |
 Box.js              |      100 |      100 |      100 |      100 |                   |
 BranchName.js       |      100 |      100 |      100 |      100 |                   |
 Button.js           |      100 |      100 |      100 |      100 |                   |
 ButtonDanger.js     |      100 |      100 |      100 |      100 |                   |
 ButtonLink.js       |      100 |      100 |      100 |      100 |                   |
 ButtonOutline.js    |      100 |      100 |      100 |      100 |                   |
 ButtonPrimary.js    |      100 |      100 |      100 |      100 |                   |
 Caret.js            |      100 |      100 |      100 |      100 |                   |
 CaretBox.js         |      100 |      100 |      100 |      100 |                   |
 CircleBadge.js      |      100 |    85.71 |      100 |      100 |                16 |
 CircleOcticon.js    |      100 |      100 |      100 |      100 |                   |
 CounterLabel.js     |      100 |      100 |      100 |      100 |                   |
 Details.js          |      100 |       80 |      100 |      100 |                14 |
 DonutChart.js       |      100 |      100 |      100 |      100 |                   |
 DonutSlice.js       |      100 |      100 |      100 |      100 |                   |
 Dropdown.js         |      100 |      100 |      100 |      100 |                   |
 FilterList.js       |      100 |      100 |      100 |      100 |                   |
 FilterListItem.js   |      100 |      100 |      100 |      100 |                   |
 Flash.js            |      100 |      100 |      100 |      100 |                   |
 FlexContainer.js    |      100 |      100 |      100 |      100 |                   |
 FlexItem.js         |      100 |      100 |      100 |      100 |                   |
 Heading.js          |      100 |      100 |      100 |      100 |                   |
 Label.js            |      100 |      100 |      100 |      100 |                   |
 Link.js             |      100 |      100 |      100 |      100 |                   |
 MergeStatus.js      |      100 |      100 |      100 |      100 |                   |
 Octicon.js          |        0 |        0 |        0 |        0 |                   |
 OcticonButton.js    |      100 |      100 |      100 |      100 |                   |
 StateLabel.js       |      100 |      100 |      100 |      100 |                   |
 Text.js             |      100 |      100 |      100 |      100 |                   |
 TextInput.js        |      100 |      100 |      100 |      100 |                   |
 Tooltip.js          |      100 |      100 |      100 |      100 |                   |
 UnderlineNav.js     |      100 |      100 |      100 |      100 |                   |
 UnderlineNavLink.js |      100 |      100 |      100 |      100 |                   |
 index.js            |        0 |        0 |        0 |        0 |                   |
 system-props.js     |      100 |      100 |      100 |      100 |                   |
 theme.js            |      100 |      100 |      100 |      100 |                   |
---------------------|----------|----------|----------|----------|-------------------|