fix failing content signature tests

[g-k]

Seeing this error https://github.com/mozilla/tls-canary/issues/227

One possible solution is pulling in the content signature JS from about remove settings / remote settings dev tools.

[g-k]

We have two integration points:

• for more complete E2E tests we would reuse the Fx RemoveSettingsClient as used in the follow clients:
  • security settings
  • normandy
  • nimbus
  • remote settings
• fetch the data directly ourselves and extract the verification code (the current approach)

After digging into this for awhile last week, RemoveSettingsClient doesn't expose a fetch method and .sync does a lot of other work (like synchronizing the local IndexDB).

Assuming verification code as used in the RS client remains, since we'll still be testing the paths Fx uses.