Missing testers: CustomField, RichTextEditor, SplitLayout, Crud, AvatarGroup, Card

Problem. Several components that are on the classpath and that users
interact with directly have no tester and therefore no generated locator, so
there is no find*() entry point for them at all. Each one has to be driven
through raw component calls, which bypasses the usability checks and the
from-client simulation the rest of the framework provides.

• CustomField — the base class for every app-defined composite field.
  Arguably the highest-value one: apps build their own fields on it constantly,
  and there's no way to set a value as a user. Related: #167.
• RichTextEditor — the user types and applies formatting; also has
  asHtml()/asDelta() value representations.
• SplitLayout — the user drags the splitter; SplitterDragendEvent.
• Crud — new/edit/delete/save/cancel, the whole point of the component.
• AvatarGroup — the user opens the overflow list.
• Card — clickable in some configurations.

Ask. Add testers for these. CustomField and RichTextEditor are the two
worth doing first. Crud is the largest and could reasonably be its own issue
if someone picks it up.

Found while auditing the project against #153 / #168.

Start by reading the existing component testers and the audit references #153 and #168, then compare how CustomField and RichTextEditor should expose find*() entry points. Implement testers for the prioritized components, with user interactions and value representations covered; Crud may be split into separate work because it is the largest component.