[Gradle, JS] Make execHandle private

#KT-38109 fixed

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/internal/testing/TCServiceMessagesTestExecutor.kt

@@ -37,7 +37,7 @@ class TCServiceMessagesTestExecutor(
     val runListeners: MutableList<KotlinTestRunnerListener>,
     val ignoreRunFailures: Boolean
 ) : TestExecuter<TCServiceMessagesTestExecutionSpec> {
-    lateinit var execHandle: ExecHandle
+    private lateinit var execHandle: ExecHandle
     var outputReaderThread: Thread? = null
     var shouldStop = false