Improve running tests in CI

Potential improvements in the CI workflow:

- use cypress github action for running cypress tests ( https://github.com/cypress-io/github-action )
- run testrpc as a service so that we don't need to start it explicitly before running the tests. Ganache-cli is available as a docker container, but it is unclear how to pass arguments to it. We would need to pass in: --mnemonic "${TEST_MNEMONIC}" --defaultBalanceEther 1000000000
- make sure we have a timeout for the cypress tests - we want to avoid that a hanging cypress test run eats up our CI minutes ( see https://github.com/cypress-io/github-action#timeouts ) - it has happened a few times for me that the tests hang
- store the test mnemonic and cypress project key in github secrets vault ( it is currently in the code )