JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/SafeCallNonNullReceiver.kt

@@ -0,0 +1,6 @@
+// http://youtrack.jetbrains.net/issue/KT-418
+
+fun ff() {
+    val i: Int = 1
+    val <!UNUSED_VARIABLE!>a<!>: Int = i<!UNNECESSARY_SAFE_CALL!>?.<!>plus(2)
+}