Make testing the updater easier

Testing the updater requires a couple of changes and some manual intervention:

update the updater URL (Firefox)
inject a different MAR certificate (Firefox)
sign mars and incremental mars if you generate them (manual steps)

I think that we could make everything more automatic, for example by providing a couple of configuration flag for the Firefox project (the custom URL and whether to replace the key), and by adding a nssdb optional configuration option in the update-response tools.

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