[K/N] Remove more ext properties

kotlin-native/backend.native/tests/build.gradle

@@ -57,6 +57,7 @@ void loadCommandLineProperties() {
 loadCommandLineProperties()
 loadLocalProperties()
 
+apply plugin: 'platform-manager'
 apply plugin: 'konan'
 apply plugin: 'kotlin'
 apply plugin: 'kotlin.native.build-tools-conventions'
@@ -71,7 +72,7 @@ configurations {
 
 ext.testOutputRoot = rootProject.file("test.output").absolutePath
 
-ext.platformManager = project.project(":kotlin-native").platformManager
+ext.platformManager = project.extensions.platformManager
 ext.target = platformManager.targetManager(project.testTarget).target
 
 ext.buildNumber = rootProject.property("kotlinVersion")