[Imperva Cloud WAF] Add system test for the CEL code

[andrewkroh]

I was reviewing the CEL code for the Imperva Cloud WAF integration, and I don't see any system test that would execute this code to ensure that it works. I only see a test for aws-s3. Let's add a mock that emulates the logs.index and log file behavior, and test against that.

There are configuration of Imperva WAF that return compressed and encrypted logs. Let's assume uncompressed and not encrypted for this test case.

References

• https://docs.imperva.com/bundle/cloud-application-security/page/settings/log-integration.htm

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[kcreddy]

Blocked by: elastic/elastic-package#1302.

[ShourieG]

Has been unblocked by elastic/elastic-package#2638