Wait for ninja proto processing to finish when exiting

Wait for the ninja proto processing goroutine to notice the fifo
has closed and exit before continuing.

Test: m nothing
Change-Id: I8cf5f3b8bf6a91496c6d2bbbd3e811eb7f0c9d21

ui/status/Android.bp

@@ -28,6 +28,7 @@ bootstrap_go_package {
     ],
     testSrcs: [
         "kati_test.go",
+        "ninja_test.go",
         "status_test.go",
     ],
 }