Rename *.jet -> *.kt in codegen tests

compiler/testData/codegen/controlStructures/continue.kt

@@ -0,0 +1,10 @@
+fun continue_test(i: Int): Int {
+  var count = i;
+  var result = 0;
+  while(count > 0) {
+    count = count - 1;
+    if (count <= 2) continue;
+    result = result + count;
+  }
+  return result;
+}