test: replace render with po in NnsProposals spec

[mstrasinskis]

Motivation

In the near future the actionable proposals will be selected by default. To be able to switch between all and actionable proposals in the spec file, they should use page object for rendering.

Changes

• Add missing PO getters.
• Switch from render() to using a page object.

Tests

• Yes.

Todos

• [ ] Add entry to changelog (if necessary). Not necessary.