chore: Implement initial block navigation tests.

[johnnesky]

Implements several unit tests described at https://docs.google.com/document/d/1pIIX5sKEG9f_NcWDIQJJEOMM7w7pQUf8eJoT3lrAElQ involving navigating with the arrow keys when starting from a block:

• Pressing down from a statement block selects its next connection.
• Pressing up from a statement block selects the previous statement's next connection.
• Pressing down from a parent block selects its input connection.
• Pressing up from a child block selects its parent's input connection.
• Pressing right from a block selects its first field.

[rachel-fenichel]

Please fix lint, then LGTM.