Cherry-pick #23239 to 7.11: Drop @timestamp for juniper.junos system tests

[marc-gr]

Cherry-pick of PR #23239 to 7.11 branch. Original message:

What does this PR do?

Configure Filebeat system tests to drop @timestamp from juniper.junos golden files.

Why is it important?

Some autogenerated logs in juniper.junos contain timestamps without a year in them, meaning that once a year the extracted timestamp will be different.

Checklist

• My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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

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Build stats

• Build Cause: Pull request #23242 opened

• Start Time: 2020-12-22T10:45:57.424+0000

• Duration: 36 min 27 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2672
Skipped	317
Total	2989

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Test stats 🧪

Test	Results
Failed	0
Passed	2672
Skipped	317
Total	2989