Add test on IPFIX writer

[jotak]

• Adding a test for the IPFIX exporter
• Small refactoring to remove some boilerplate code and have all fields more easily accessible

cc @bhale , I believe this test can help you when adding more ipfix fields

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 35 lines in your changes are missing coverage. Please review.

Project coverage is 67.03%. Comparing base (07b7a3e) to head (525f0af). Report is 1 commits behind head on main.

Files	Patch %	Lines
pkg/pipeline/write/write_ipfix.go	0.00%	35 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #606 +/- ## ========================================== + Coverage 66.09% 67.03% +0.93% ========================================== Files 103 103 Lines 7509 7401 -108 ========================================== - Hits 4963 4961 -2 + Misses 2254 2147 -107 - Partials 292 293 +1 ``` | [Flag](https://app.codecov.io/gh/netobserv/flowlogs-pipeline/pull/606/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=netobserv) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/netobserv/flowlogs-pipeline/pull/606/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=netobserv) | `67.03% <0.00%> (+0.93%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=netobserv#carryforward-flags-in-the-pull-request-comment) to find out more.

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[jotak]

/approve

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

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