[ci] Add PR it-tests (part 1+2) to Buildkite (#15438)

This commit adds integration tests to the
Buildkite PR pipeline.

Relates:

- #15437
- elastic/ingest-dev#1721
- #15279

.buildkite/pull_request_pipeline.yml

@@ -42,3 +42,33 @@ steps:
       source .buildkite/scripts/common/container-agent.sh
       source .buildkite/scripts/pull-requests/sonar-env.sh
       ci/unit_tests.sh java
+
+  - label: ":lab_coat: Integration Tests / part 1"
+    key: "integration-tests-part-1"
+    agents:
+      image: "docker.elastic.co/ci-agent-images/platform-ingest/buildkite-agent-logstash-ci-no-root"
+      cpu: "8"
+      memory: "16Gi"
+      ephemeralStorage: "100Gi"
+      # Run as a non-root user
+      imageUID: "1002"
+    command: |
+      set -euo pipefail
+
+      source .buildkite/scripts/common/container-agent.sh
+      ci/integration_tests.sh split 0
+
+  - label: ":lab_coat: Integration Tests / part 2"
+    key: "integration-tests-part-2"
+    agents:
+      image: "docker.elastic.co/ci-agent-images/platform-ingest/buildkite-agent-logstash-ci-no-root"
+      cpu: "8"
+      memory: "16Gi"
+      ephemeralStorage: "100Gi"
+      # Run as a non-root user
+      imageUID: "1002"
+    command: |
+      set -euo pipefail
+
+      source .buildkite/scripts/common/container-agent.sh
+      ci/integration_tests.sh split 1

.buildkite/scripts/common/container-agent.sh

@@ -7,5 +7,11 @@
 
 set -euo pipefail
 
-export PATH="/usr/local/rbenv/bin:$PATH"
-eval "$(rbenv init -)"
+if [[ $(whoami) == "logstash" ]]
+then
+    export PATH="/home/logstash/.rbenv/bin:$PATH"
+    eval "$(rbenv init -)"
+else
+    export PATH="/usr/local/rbenv/bin:$PATH"
+    eval "$(rbenv init -)"
+fi