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

compiler/testData/codegen/controlStructures/break.kt

@@ -0,0 +1,10 @@
+fun fac(i: Int): Int {
+    var count = 1;
+    var result = 1;
+    while(true) {
+        count = count + 1;
+        result = result * count;
+        if (count == i) break;
+    }
+    return result;
+}