JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/dataFlowInfoTraversal/When.kt

@@ -0,0 +1,18 @@
+fun bar(x: Int): Int = x + 1
+
+fun foo() {
+    val x: Int? = null
+
+    if (x != null) {
+        when (x) {
+            0 -> bar(x)
+            else -> {}
+        }
+    }
+
+    when (x) {
+        0 -> { if (<!SENSELESS_COMPARISON!>x == null<!>) return }
+        else -> { if (x == null) return }
+    }
+    bar(x)
+}