[sda-download] refactor test scripts

[jbygdell]

Please describe the feature

As a developer, I want the test scripts to be logical, both i how they are named and what they test so that I don't have to spend time on figuring out why test X failed when I don't expect it to.

Acceptance criteria

• Split the bash scripts into tests for the separate endpoints for sda-download
  • metadata
  • REST file download, separate tests for decrypred and encrypted data.
  • "S3" endpoint, separate tests for decrypred and encrypted data.
• Any script should be able to run in isolation

Additional context

Estimation of size

medium

Estimation of priority

medium