Slow imagescan unit tests

[anjmao]

These tests are really slow, check previous pipelines and for more examples.

ok github.com/castai/kvisor/cmd/imagescan/collector 165.970s

[samuelvl]

I reworked the tests as part of this PR #556. The new execution time when running tests locally is:

ok  	github.com/castai/kvisor/cmd/imagescan/collector	3.567s