JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/nullabilityAndAutoCasts/kt2109.kt

@@ -0,0 +1,20 @@
+//KT-2109 Nullability inference fails in extension function
+package kt2109
+
+class A {
+    fun foo() {}
+}
+
+fun A?.bar() {
+    if (this == null) {
+        return
+    }
+    foo()
+}
+
+fun A.baz() {
+    if (<!SENSELESS_COMPARISON!>this == null<!>) {
+        return
+    }
+    foo()
+}