SKA: Update and breakdown x-pack/.gitignore (elastic#212341)

## Summary
* Removes unneeded entries from `x-pack/.gitignore`.
* Moves `x-pack/test/` entries to a new, more specific file.

.github/CODEOWNERS

@@ -1600,6 +1600,7 @@ packages/kbn-monaco/src/esql @elastic/kibana-esql
 /.eslintignore @elastic/kibana-operations
 
 # QA - Appex QA
+/x-pack/test/.gitignore @elastic/appex-qa
 /src/platform/packages/shared/kbn-es/src/serverless_resources/project_roles/es/roles.yml @elastic/appex-qa
 /src/platform/packages/shared/kbn-es/src/serverless_resources/project_roles/oblt/roles.yml @elastic/appex-qa
 /src/platform/packages/shared/kbn-es/src/serverless_resources/project_roles/security/roles.yml @elastic/appex-qa

x-pack/.gitignore

@@ -1,19 +1,6 @@
 /.gradle
 /build
 /target
-/test/functional/failure_debug
-/test/functional/screenshots
-/test/functional/apps/**/reports/session
-/test/reporting/configs/failure_debug/
-
-# TODO remove once all of the modules have been relocated
-# we keep them around to avoid conflicts derived of merge commits,
-# suddenly adding a huge amount of files due to these rules being outdated
-/platform/plugins/private/reporting/.chromium/
-/platform/plugins/private/reporting/.phantom/
-/plugins/screenshotting/.chromium/
-/plugins/screenshotting/chromium/
-
 /.aws-config.json
 /.env
 /.kibana-plugin-helpers.dev.*

x-pack/test/.gitignore

@@ -0,0 +1,4 @@
+/functional/failure_debug
+/functional/screenshots
+/functional/apps/**/reports/session
+/reporting/configs/failure_debug/