Fix building and testing on macs (#380)

Removing the Nx packages broke the build step on arm processors, due to
the bug in npm with optional architecture dependencies. The same bug
I've ran into several times when updating the Nx packages. This fixes
those missing dependencies (had to delete node_modules and package-lock.

Also, running the tests after removing Nx, in a clean repo, failed on
TypeErrors with getTextBy. This adds a temporary fix by downgrading
testing-library/react to the previous major version.