Additionally - remove RestbaseInterface::checkBackend.
When HTTP requests are banned from unit tests this method
makes no sence.
MathoidCli tests were not reenabled cause it's a bit hard
to mock shell acceess. Hopefully as part of Shellbox
transition we can have MockShell trait in core and reenable
those tests
Bug: T265628
Depends-On: I350ed59af603a0504704af3265787a4be8f2dc2a
Change-Id: I294d0ed905193850dc38cb3071eb9b31c8863121
