Clean up unit test warnings

[ThomasKindermann]

When running yarn.test several warnings are thrown.

I want to reduce them to the bare minimum

Missing Key for list item

• ArrayWidget.test.js
• AlignWidget.test.jsx
• LeadImageSidebar.test.jsx
• More.test.jsx
• Pluggable.test.js

Failed propType:

• ListingView.test.jsx
• TabularView.test.jsx
• SummaryView.test.jsx
• Edit.test.jsx
• NumberWidget.test.jsx
• ContentsItem.test.jsx
• PasswordReset.test.jsx
• Contents.test.js

Function components cannot be given refs:

• Controlpanel.test.jsx
• Edit.test.jsx
• ContentType.test.jsx

intl Errors:

• Contents.test.jsx

componentWillReceiveProps has been renamed:

• DateRangeFacet.test.jsx
• DatetimeWidget.test.jsx

Other

• warn: default extension
• withBlockSchemaEnhancer.test.js (i dont know how to set default extension)

[ThomasKindermann]

PR: #3775

[sneridagh]

Since we are working in #6605 this might become obsolete.