JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/controlStructures/kt786.kt

@@ -0,0 +1,26 @@
+package kt786
+
+//KT-786 Exception on incomplete code with 'when'
+fun foo() : Int {
+    val d = 2
+    var z = 0
+    when(d) {
+        is 5, is 3 -> <!UNUSED_CHANGED_VALUE!>z++<!>
+        <!ELSE_MISPLACED_IN_WHEN!>else<!> -> { z = <!UNUSED_VALUE!>-1000<!> }
+        <!UNREACHABLE_CODE!>return z -> 34<!>
+    }
+<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+
+//test unreachable code
+fun fff(): Int {
+    var d = 3
+    <!NO_ELSE_IN_WHEN!>when<!>(d) {
+        is 4 -> 21
+        return 2 -> <!UNREACHABLE_CODE!>return 47<!>
+        <!UNREACHABLE_CODE!>bar() -> 45<!>
+        <!UNREACHABLE_CODE!>444 -> true<!>
+    }
+    return 34
+}
+
+fun bar(): Int = 8