Cherry-pick #22107 to 7.x: [libbeat] Add more disk queue unit tests and fix a size-check bug

[faec]

Cherry-pick of PR #22107 to 7.x branch. Original message:

What does this PR do?

Add more unit tests to the disk queue, and fix a bug they uncovered where data in an active write request was not counted against the total queue size when checking capacity.

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/integrations (Team:Integrations)

[elasticmachine]

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Build stats

• Build Cause: [Pull request #22112 opened]

• Start Time: 2020-10-22T20:05:36.667+0000

• Duration: 98 min 31 sec

Test stats 🧪

Test	Results
Failed	0
Passed	16341
Skipped	1365
Total	17706