[docs-patrol] Update integration testing docs for TestMain auto-build

Recent code changes in the repository have introduced documentation drift. The following changes need corresponding documentation updates.

## Changes Requiring Documentation Updates

### 1. Go integration tests now auto-build test binaries via `TestMainWithBuild`, but docs still require `mage buildSystemTestBinary`

**Commit(s):**
- 323f731f84e5d8e55a1bc3248b568097dbb402bb ([link](https://github.com/elastic/beats/commit/323f731f84e5d8e55a1bc3248b568097dbb402bb))

**What changed:**
- `filebeat/magefile.go` marks `BuildSystemTestBinary` as deprecated for Go integration tests and prints:
- `"The test binary is now built automatically via TestMain."`
- Go integration tests now call `integration.TestMainWithBuild(m, "filebeat")` in `filebeat/tests/integration/integration_test.go`.
- `libbeat/tests/integration/framework.go` implements `TestMainWithBuild` and performs `testbin.Build(...)` automatically before running tests.

**Documentation impact:**
- `docs/extend/testing.md` still presents `mage buildSystemTestBinary` as a required step for Go integration tests (`# Build the test binary` before `go test ... -tags integration`).
- `docs/extend/testing.md` also states: `To create the testing binary run mage buildSystemTestBinary.`
- `filebeat/input/journald/README.md` still instructs manual test flow with `mage buildSystemTestBinary` before the Go integration test command.

These instructions are now outdated for Go integration tests and should be updated to avoid unnecessary/incorrect setup steps.

## Suggested Actions

- [ ] Update `docs/extend/testing.md` Go integration section to state that test binaries are auto-built via `TestMainWithBuild`.
- [ ] Clarify in `docs/extend/testing.md` that `mage buildSystemTestBinary` is only required for Python system-test workflows (if applicable), not Go integration tests.
- [ ] Update `filebeat/input/journald/README.md` manual integration-test instructions to remove (or explicitly re-scope) the `mage buildSystemTestBinary` prerequisite for Go integration tests.

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> - [x] expires on Apr 23, 2026, 2:53 PM UTC

Read docs/extend/testing.md and filebeat/input/journald/README.md, then compare their Go integration instructions with TestMainWithBuild in filebeat/tests/integration/integration_test.go and libbeat/tests/integration/framework.go. Update the two documentation flows to reflect automatic test-binary builds and keep mage buildSystemTestBinary scoped correctly. Done means the Go instructions no longer require an unnecessary manual build step.

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