[ci] Add testing phase to exhaustive tests suite (#15711) (#15718)

This is the second part of the migration of the exhaustive/main
Jenkins Job to Buildkite. So far we've migrated the "compatibility
phase" and this commit adds the "testing phase"[^1], which is essentially
the same amount of tests that we ran on PR jobs.

Relates elastic/ingest-dev#1722
Depends #15708

[^1]: For more details, refer to the sequence diagram in elastic/ingest-dev#1722 (comment)

(cherry picked from commit 9538338)

Co-authored-by: Dimitrios Liappis <[email protected]>

.buildkite/scripts/exhaustive-tests/generate-steps.py

@@ -9,6 +9,7 @@
 
 VM_IMAGES_FILE = ".buildkite/scripts/common/vm-images.json"
 VM_IMAGE_PREFIX = "platform-ingest-logstash-multi-jdk-"
+CUR_PATH = os.path.dirname(os.path.abspath(__file__))
 
 def slugify_bk_key(key: str) -> str:
     """
@@ -20,6 +21,10 @@ def slugify_bk_key(key: str) -> str:
 
     return key.translate(mapping_table)
 
+def testing_phase_steps() -> typing.Dict[str, typing.List[typing.Any]]:
+    with open(os.path.join(CUR_PATH, "..", "..", "pull_request_pipeline.yml")) as fp:
+        return YAML().load(fp)
+
 def compat_linux_step(imagesuffix: str) -> dict[str, typing.Any]:
     linux_command = LiteralScalarString("""#!/usr/bin/env bash
 set -eo pipefail
@@ -89,15 +94,23 @@ def randomized_windows_os() -> str:
 
     structure = {"steps": []}
 
+    structure["steps"].append({
+        "group": "Testing Phase",
+        "key": "testing-phase",
+        **testing_phase_steps(),
+    })
+
     structure["steps"].append({
             "group": "Compatibility / Linux",
             "key": "compatibility-linux",
+            "depends_on": "testing-phase",
             "steps": compat_linux_steps,
     })
 
     structure["steps"].append({
             "group": "Compatibility / Windows",
             "key": "compatibility-windows",
+            "depends_on": "testing-phase",
             "steps": [compat_windows_step(imagesuffix=windows_test_os)],
     })