Fix flaky OpenSearchTestBasePluginFuncTest (#20955)

When integTest runs with maxParallelForks > 1, all parallel test JVMs
share the same default .gradle-test-kit directory. This causes a race
condition in Gradle 9.4 where concurrent tests simultaneously read and
write the Groovy DSL compiled script cache (cp_settings), resulting in
a NoSuchFileException and UnexpectedBuildFailure.

Fix by assigning each test method its own isolated TestKit directory
via withTestKitDir() using a subdirectory within the already-unique
TemporaryFolder. This eliminates cross-test cache contention while
preserving daemon and task history reuse within the same test method
(required for UP_TO_DATE assertions).

Signed-off-by: Dharmesh <dharmesh.singh@uber.com>

Author: psychbot

buildSrc/src/integTest/groovy/org/opensearch/gradle/fixtures/AbstractGradleFuncTest.groovy

@@ -45,11 +45,13 @@ abstract class AbstractGradleFuncTest extends Specification {
 
     File settingsFile
     File buildFile
+    File testKitDir
 
     def setup() {
         settingsFile = testProjectDir.newFile('settings.gradle')
         settingsFile << "rootProject.name = 'hello-world'\n"
         buildFile = testProjectDir.newFile('build.gradle')
+        testKitDir = testProjectDir.newFolder('.gradle-test-kit')
     }
 
     GradleRunner gradleRunner(String... arguments) {
@@ -62,6 +64,7 @@ abstract class AbstractGradleFuncTest extends Specification {
                 .withProjectDir(projectDir)
                 .withArguments(arguments)
                 .withPluginClasspath()
+                .withTestKitDir(testKitDir)
                 .forwardOutput()
     }