Add initial e2e specs using new test infrastructure

[whymarrh]

This PR adds the first couple of test specs using the new e2e test infrastructure. These tests display the ability to use pre-existing (fixture)data for both the extension itself and Ganache (as added in #6215).

Example runs

$ npx mocha --no-timeouts e2e/tests/**/*.spec.js


  MetaMask Browser Extension first-time flow
    ✓ displays a welcome screen with a clickable continue button (11466ms)
    ✓ can on-board by creating a new wallet (16684ms)

  MetaMask Browser Extension from existing state
    ✓ displays the lock screen (11063ms)


  3 passing (39s)

Test cases

• Simple first-time flow process (no pre-existing app state, no Ganache state)
• Simple view transaction (fully-onboarded app state, no Ganache state)
• View pre-existing list of transactions from Ganache (pre-existing app & Ganache state)

[whymarrh]

I'm closing this for now as this work is being split into smaller more concrete PRs