[yarn] Introduce yarn jest-all to run jest in all workspaces

Summary:
Runs jest unit tests in workspaces that were previously being tested in eslint_flow_jest.yml workflow.

Reduces length of command in eslint_flow_jest.yml workflow and may be generally helpful.

Test Plan:
Works as expected:
https://blob.sh/1ce144.png

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6650