Remove the `test-big.html` file from the repository and instead generate it using a script

[Greenheart]

The first time the tests are run locally, we could generate a random file which would then be used to test encryption of large files. This file would be cached locally (ignored by git) and used to verify the decrypted output.

This would be suitable to run in a test setup step, and only if the file test-big.html doesn't exist yet.

This will be easy to implement once we fix #56