[zeek] Add system tests for Splunk inputs

[marc-gr]

What does this PR do?

• Adds system tests for the Splunk inputs along with the required fixes to the pipeline.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
  - [ ] If I'm introducing a new feature, I have modified the Kibana version constraint in my package's manifest.yml file to point to the latest Elastic stack release (e.g. ^7.13.0).

Related issues

• Relates Move Security integrations to GA #961

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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Build stats

• Build Cause: Pull request #1108 updated

• Start Time: 2021-06-09T14:43:44.605+0000

• Duration: 65 min 48 sec

• Commit: 3b7b5a4

Test stats 🧪

Test	Results
Failed	0
Passed	191
Skipped	0
Total	191

Trends 🧪

[marc-gr]

/test

[P1llus]

LGTM