FEATURE: show/hide components attribute

[stevebuik]

It's often useful to hide the components being tested so that only the status bar, assertions and failures are displayed in the UI.

An attribute could be added with a checkbox in the status bar to toggle it. The value can be a css string which toggles slds-hide in and out of the string. This can then be used in the markup to wrap the test markup.

The checkbox will need to fire a toggle component event to the test since it is a child component.