Add unit tests for vulnerability report and fix first_seen

[dekkers]

Changes

This adds unit tests for the vulnerability report. I spend some time figuring out how to best mock the octopoes calls. I created a a mock octopoes connector class on which you can set a dictionary with queries and the results that should be returned. Suggestions on how to this better / make it easier to write tests are welcome.

It also fixes that first_seen of the previous finding was used if the finding didn't have a history (I spotted this while reading the code to write the tests).

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Checklist for code reviewers:

Copy-paste the checklist from the docs/source/templates folder into your comment.

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Checklist for QA:

Copy-paste the checklist from the docs/source/templates folder into your comment.

[underdarknl]

Lets implement last seen in a new ticket.

[stephanie0x00]

Checklist for QA:

• I have checked out this branch, and successfully ran a fresh make reset.
• I confirmed that there are no unintended functional regressions in this branch:
  • I have managed to pass the onboarding flow
  • Objects and Findings are created properly
  • Tasks are created and completed properly
• I confirmed that the PR's advertised feature or hotfix works as intended.

What works:

All seems to work well, ready for merge.

What doesn't work:

n/a

Bug or feature?:

n/a