Add react native unit tests to ci

[tafsiri]

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[tafsiri]

These are unit tests that run in the browser and serve as a quicker sanity check against api breakage wrt to core.

Also add some documentation to cloudbuild.yml

cc @annxingyuan I updated the cloudbuild.yml to make the webgpu tests not wait on yarn install of core (since it has its own yarn install). Let me know if you see any foresee any problem with this.

[nsthorat]

CI seems to be failing on yarn test but it seems to be successful, any idea why?

[nsthorat]

Reviewed 5 of 5 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @dsmilkov)

[tafsiri]

Hmm, failed after i synced with master. Step #2 - "test": Chrome 76.0.3809 (Windows 10.0.0) gramSchmidt-non-tiny webgl2 {"WEBGL_CPU_FORWARD":false,"WEBGL_SIZE_UPLOAD_UNIFORM":0} 16x32 FAILED it failed with a timeout so maybe it was just slow to respond on that run? I'll kick the build again.

[tafsiri]

@nsthorat passing now, i think its just a slow test.

[dsmilkov]

Reviewed 5 of 5 files at r1.
Reviewable status: complete! 2 of 1 approvals obtained (waiting on @dsmilkov)