JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/nullabilityAndAutoCasts/kt2223.kt

@@ -0,0 +1,8 @@
+//KT-2223 Comparing non-null value with null might produce helpful warning
+package kt2223
+
+fun foo() {
+    val x: Int? = null
+    if (x == null) return
+    if (<!SENSELESS_COMPARISON!>x == null<!>) return
+}