Optimise e2e tests retries

Summary:Implemented smarter retries for e2e tests

E2e tests consist of two expensive and flaky steps: npm installation and tests execution.
Our CI is configured to retry a command 3 times before terminating the test.
In current setup if one of the steps fail the whole test is restarted.

This change adds ad-hoc ability to retry flaky bits of e2e script independently.
This will make tests fail faster when code gets broken while increasing the chances to succeed in case of random false errors.
Closes https://github.com/facebook/react-native/pull/7184

Differential Revision: D3218927

fb-gh-sync-id: 9be8343484bb28aa3601b651db70fc55aa840e61
fbshipit-source-id: 9be8343484bb28aa3601b651db70fc55aa840e61