Add SolidUI test helpers

[atangster]

Extend solid_test with helpers for testing solidui widget interactions.

Example helpers to create:

1. SolidLoginHelper - enterWebId(), enterPassword(), performLogin()
2. SolidScaffoldHelper - selectMenuItem(), tapAppBarAction(), openOverflowMenu()
3. SolidAuthHelper - performLogout(), dismissSecurityKeyPopup()
4. SolidStatusBarHelper - tapLoginStatus(), getDisplayedWebId()