[elasticsearch] update slowlog test files

[klacabane]

Related #35133

Leading newlines in slowlog's source object are not supported anymore. This change updates the test files to the updated multiline format introduced in elastic/elasticsearch#38081. See additional context #35133 (comment)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @klacabane? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-04-21T10:24:13.099+0000

• Duration: 71 min 37 sec

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Test	Results
Failed	0
Passed	7693
Skipped	753
Total	8446

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[klacabane]

Merge to unblock #34589. If any follow ups are needed, we'll add them in separate PR