[mimecast] Add content-disposition to test logs

[djordje-adzemovic-devtech]

What does this PR do?

Add content-disposition to test mock to properly create sample event from SIEM logs.

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.
• I have verified that Kibana version constraints are current according to guidelines.

[elasticmachine]

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• Start Time: 2022-02-24T09:25:02.915+0000

• Duration: 30 min 28 sec

Test stats 🧪

Test	Results
Failed	0
Passed	61
Skipped	0
Total	61

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[elasticmachine]

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

[andrewkroh]

/test

[andrewkroh]

/test

[marc-gr]

/test

[marc-gr]

/test