Populate settings also for buildSrc module.

^KT-45745 In Progress

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/testbase/testDsl.kt

@@ -259,16 +259,27 @@ private fun setupProjectFromTestResources(
 private fun Path.addDefaultBuildFiles() {
     if (Files.exists(resolve("build.gradle"))) {
         val settingsFile = resolve("settings.gradle")
-        if (!Files.exists(settingsFile)) {
+        settingsFile.addPluginManagementToSettings()
-            settingsFile.toFile().writeText(DEFAULT_GROOVY_SETTINGS_FILE)
+    }
+
+    val buildSrc = resolve("buildSrc")
+    if (Files.exists(buildSrc)) {
+        val settingsFile = buildSrc.resolve("settings.gradle")
+        settingsFile.addPluginManagementToSettings()
+    }
+}
+
+private fun Path.addPluginManagementToSettings() {
+    if (!Files.exists(this)) {
+        toFile().writeText(DEFAULT_GROOVY_SETTINGS_FILE)
     } else {
-            val settingsContent = settingsFile.toFile().readText()
+        val settingsContent = toFile().readText()
         if (!settingsContent
                 .lines()
                 .first { !it.startsWith("//") }
                 .startsWith("pluginManagement {")
         ) {
-                settingsFile.toFile().writeText(
+            toFile().writeText(
                 """
                     $DEFAULT_GROOVY_SETTINGS_FILE
                     
@@ -278,7 +289,6 @@ private fun Path.addDefaultBuildFiles() {
         }
     }
 }
-}
 
 private fun TestProject.agreeToBuildScanService() {
     val settingsFile = if (Files.exists(settingsGradle)) settingsGradle else settingsGradleKts