JetDiagnosticsTest migrated to TestGenerator

- test data files renamed from *.jet to *.kt

compiler/testData/diagnostics/tests/regressions/kt860.kt

@@ -0,0 +1,14 @@
+// KT-860 ConcurrentModificationException in frontend
+
+package kotlin.util
+
+import java.util.*
+import jet.Iterator
+
+fun <T, U: Collection<in T>> Iterator<T>.to(container: U) : U {
+    while(hasNext)
+        container.add(next())
+    return container
+}
+
+inline fun <T> Iterator<T>.toArrayList() = to(ArrayList<T>())