Add a helper function to MockService to accept overloads

[satanTime]

instead of

const ngbModalRef = MockService(NgbModalRef);
ngbModalRef.componentInstance = certificatesFormMock;
ngbModalRef.close = closeSpy;

to have an option like

const ngbModalRef = MockService(NgbModalRef);
ngbModalRef.__override({
  componentInstance: certificatesFormMock,
  close: closeSpy,
});

ensure that it supports readonly properties and methods too.