Fix JDK matrix pipeline after configurable it split (#17461)

PR #17219 introduced configurable split quantities for IT tests, which
resulted in broken JDK matrix pipelines (e.g. as seen via the elastic
internal link:
https://buildkite.com/elastic/logstash-linux-jdk-matrix-pipeline/builds/444

reporting the following error

```
  File "/buildkite/builds/bk-agent-prod-k8s-1743469287077752648/elastic/logstash-linux-jdk-matrix-pipeline/.buildkite/scripts/jdk-matrix-tests/generate-steps.py", line 263
    def integration_tests(self, part: int, parts: int) -> JobRetValues:
    ^^^
SyntaxError: invalid syntax
There was a problem rendering the pipeline steps.
Exiting now.
```
)

This commit fixes the above problem, which was already fixed in #17642, using a more
idiomatic way.

Co-authored-by: Andrea Selva <selva.andre@gmail.com>

Author: dliappis

.buildkite/scripts/jdk-matrix-tests/generate-steps.py

@@ -4,6 +4,7 @@
 import os
 import sys
 import typing
+from functools import partial
 
 from ruamel.yaml import YAML
 from ruamel.yaml.scalarstring import LiteralScalarString
@@ -257,8 +258,8 @@ def ruby_unit_test(self) -> JobRetValues:
             retry=copy.deepcopy(ENABLED_RETRIES),
         )
 
-    def integration_test_parts(self, parts) -> list[JobRetValues]:
-        return list(map(lambda idx: integration_tests(self, idx+1, parts), range(parts)))
+    def integration_test_parts(self, parts) -> list[partial[JobRetValues]]:
+        return [partial(self.integration_tests, part=idx+1, parts=parts) for idx in range(parts)]
 
     def integration_tests(self, part: int, parts: int) -> JobRetValues:
         step_name_human = f"Integration Tests - {part}/{parts}"
@@ -276,8 +277,8 @@ def integration_tests(self, part: int, parts: int) -> JobRetValues:
             retry=copy.deepcopy(ENABLED_RETRIES),
         )
 
-    def pq_integration_test_parts(self, parts) -> list[JobRetValues]:
-        return list(map(lambda idx: pq_integration_tests(self, idx+1, parts), range(parts)))
+    def pq_integration_test_parts(self, parts) -> list[partial[JobRetValues]]:
+        return [partial(self.pq_integration_tests, part=idx+1, parts=parts) for idx in range(parts)]
 
     def pq_integration_tests(self, part: int, parts: int) -> JobRetValues:
         step_name_human = f"IT Persistent Queues - {part}/{parts}"