Improve testing instructions

[chrisdunnbirch]

Problem

It is currently difficult to figure out how to run the yarn test:integration suite because there are some undocumented hoops to jump through before this will work as expected.

Suggested solution

1. Add .env.sample to committed files, this should include the requisite variable names with no values so that a new contributor knows which values to provide
2. Add op run --env-file=.env -- to the start of test:integration package script to reduce complexity for contributors
3. Add brief instructions to README.md outlining requirements for running test suites (copy .env.sample, add account/vault values, etc)