[Gradle] Migrate functional tests from Project.buildDir usage

It's going to be deprecated in Gradle 8.3
^KTI-1473 In Progress

libraries/tools/kotlin-gradle-plugin/src/functionalTest/kotlin/org/jetbrains/kotlin/gradle/regressionTests/KT31459JvmMainClassesTest.kt

@@ -56,7 +56,7 @@ class KT31459JvmMainClassesTest {
             // make copy of original classes directory
             val originalClassesDirs: FileCollection =
                 project.files(classesDirs.from.toTypedArray())
-            val transformedClassesDir = project.buildDir.resolve("classes/atomicfu/${target.name}/${compilation.name}")
+            val transformedClassesDir = project.layout.buildDirectory.dir("classes/atomicfu/${target.name}/${compilation.name}")
             val transformTask = project.tasks.create("transformTask") {
                 it.dependsOn(compilation.compileAllTaskName)
                 it.inputs.files(originalClassesDirs)

libraries/tools/kotlin-gradle-plugin/src/functionalTest/kotlin/org/jetbrains/kotlin/gradle/unitTests/CInteropCommonizerArtifactTypeAttributeTest.kt

@@ -24,7 +24,7 @@ class CInteropCommonizerArtifactTypeAttributeTest {
         val project = buildProjectWithMPP()
 
         /* Create stub klibs collection directory */
-        val klibCollectionDir = project.buildDir.resolve("testOutputDir")
+        val klibCollectionDir = project.layout.buildDirectory.dir("testOutputDir").get().asFile
         klibCollectionDir.mkdirs()
 
         val klibs = listOf(