Fix translations text not being found by getByText

[pierrezimmermannbam]

Summary

Pr is still a draft. Fixes #937

Test plan

[mdjastrzebski]

This issue is related to architecture discussion in #972.

ReactTestRendererNode seems to be better suited for handling text related stuff, but otherwise we rely on ReactTestInstance it would be great to have an easy way of getting ReactTestRendererNode from ReactTestInstance. It seems that React Test Renderer source has such function but unfortunately it is not exported and we are limited to calling toJSON on ReactTestRenderer instance.

[mdjastrzebski]

Closing as changes contained it the PR would break other features of RNTL.