Cherry-pick #22177 to 7.10: Skip flaky gateway tests

[michalpristas]

Cherry-pick of PR #22177 to 7.10 branch. Original message:

What does this PR do?

This test relies on timing and is flaky when CI is slow.

Why is it important?

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.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[blakerouse]

Backport looks good.

[elasticmachine]

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	1382
Skipped	6
Total	1388

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #22205 updated]

• Start Time: 2020-11-03T07:01:54.360+0000

• Duration: 38 min 10 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1382
Skipped	6
Total	1388