[8.19](backport #49583) filebeat: auto-build test binary in Go integration tests via TestMain

[mergify]

Proposed commit message

filebeat: auto-build test binary in Go integration tests via TestMain

Move the filebeat.test binary build step from the mage targets into TestMain so that `go test -tags integration` works without requiring a prior `mage buildSystemTestBinary`. This removes a manual prerequisite that tripped up developers running integration tests locally.

  - Add BuildSystemTestBinary helper to libbeat/tests/integration/framework.go
  - Add TestMain in filebeat and x-pack/filebeat integration test packages
  - Remove BuildSystemTestBinary from mage GoIntegTest/GoFIPSOnlyIntegTest
  - Python integration tests are unchanged (they still use devtools directly)

Assisted by Claude

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. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• [ ] I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

• N/A

How to test this PR locally

• Ensure there is not "SystemTestBinary", run any filebeat Go integration test, they'll works

Related issues

• N/A

  ---

  This is an automatic backport of pull request filebeat: auto-build test binary in Go integration tests via TestMain #49583 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   dev-tools/mage/gotest.go
	new file:   dev-tools/testbin/testbin.go
	modified:   filebeat/magefile.go
	new file:   filebeat/tests/integration/integration_test.go
	renamed:    filebeat/input/filestream/legacy_metrics_integration_test.go -> filebeat/tests/integration/legacy_metrics_integration_test.go
	modified:   libbeat/magefile.go
	new file:   libbeat/tests/integration/integration_test.go
	modified:   metricbeat/magefile.go
	new file:   metricbeat/tests/integration/integration_test.go
	modified:   x-pack/filebeat/magefile.go
	new file:   x-pack/filebeat/tests/integration/integration_test.go
	renamed:    x-pack/filebeat/input/gcppubsub/otel_test.go -> x-pack/filebeat/tests/integration/otel_gcppubsub_test.go
	new file:   x-pack/filebeat/tests/integration/testdata/gcp_pubsub_fake_credentials.json
	modified:   x-pack/libbeat/magefile.go
	new file:   x-pack/libbeat/tests/integration/integration_test.go
	modified:   x-pack/metricbeat/magefile.go
	new file:   x-pack/metricbeat/tests/integration/integration_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   libbeat/tests/integration/framework.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

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[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)