Changed before for beforeAll, refactored Bouncer tests.

[nventuro]

Fixes #1021.

Some before functions remain so that time can be measured properly:

before(async function () {
  // Advance to the next block to correctly read time in the solidity "now" function interpreted by ganache
  await advanceBlock();
});

Ideally the helpers would take care of this automatically, but I'm not sure how we'd go about implementing it.

I also refactored the Bouncer tests, since they relied heavily on the before block and reused contract state, and improved readability and coverage in the process.

[frangio]

Unfortunately the SignatureBouncer tests had been changed quite a lot in #1024. Can you cherry pick the relevant commits here since that PR is on hold for now?

[nventuro]

Done! I brought in the helpers/sign.js changes from b85d8d6.

[shrugs]

hopefully the merge works well, but should be fine after a rebase. LGMT 👍