Build: fix eclipse after icTests (#56362)

We made a small mistake when breaking out the `ESIntegTestCase`
subclasses that confused eclipse. This makes it happy again. Poor
eclipse!

Relates #55896

Author: nik9000

buildSrc/src/main/java/org/elasticsearch/gradle/util/GradleUtils.java

@@ -41,6 +41,7 @@
 import org.gradle.plugins.ide.eclipse.model.EclipseModel;
 import org.gradle.plugins.ide.idea.model.IdeaModel;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
@@ -170,7 +171,12 @@ public static TaskProvider<?> addTestSourceSet(Project project, String sourceSet
         });
         project.getPluginManager().withPlugin("eclipse", p -> {
             EclipseModel eclipse = project.getExtensions().getByType(EclipseModel.class);
-            eclipse.getClasspath().setSourceSets(List.of(testSourceSet));
+            List<SourceSet> eclipseSourceSets = new ArrayList<>();
+            for (SourceSet old : eclipse.getClasspath().getSourceSets()) {
+                eclipseSourceSets.add(old);
+            }
+            eclipseSourceSets.add(testSourceSet);
+            eclipse.getClasspath().setSourceSets(sourceSets);
             eclipse.getClasspath().getPlusConfigurations().add(runtimeClasspathConfiguration);
         });