[ci] Reusable unit + IT test steps for Buildkite (#15708) (#15712)

This commit is a pre-requisite for adding unit + IT tests in a
dedicated phase of the Exhaustive tests pipeline.

It refactors the tests currently used by PR jobs, so that they become
reusable.

(cherry picked from commit 03d7b59f2a)

Co-authored-by: Dimitrios Liappis <dimitrios.liappis@gmail.com>

.buildkite/pull_request_pipeline.yml

@@ -38,9 +38,14 @@ steps:
       ephemeralStorage: "100Gi"
     command: |
       set -euo pipefail
+      if [[ $BUILDKITE_PULL_REQUEST == "false" ]]; then
+        # https://github.com/elastic/logstash/pull/15486 for background
+        export ENABLE_SONARQUBE="false"
+      else
+        source .buildkite/scripts/pull-requests/sonar-env.sh
+      fi
 
       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"