Bug 1257581 - Fix and re-enable downloads misc tests. r=kmag

The original problem was that the erase() test did not wait for downloads
to complete which caused a race with cleanup code trying to remove the
download directory while downloads were still writing to it.  This was
fixed in https://hg.mozilla.org/mozilla-central/rev/d6d9e7411319

But meanwhile, the removeFile() test bit-rotted slightly, fixed it here
and re-enabled the misc test.

MozReview-Commit-ID: 6BgDKKkUK55

--HG--
extra : rebase_source : 29eef70201ff776c637bc0bb98a98b48e26907ca

toolkit/components/extensions/test/mochitest/chrome.ini

@@ -7,7 +7,6 @@ support-files =
 
 [test_chrome_ext_downloads_download.html]
 [test_chrome_ext_downloads_misc.html]
-skip-if = 1 # Currently causes too many intermittent failures.
 [test_chrome_ext_downloads_search.html]
 [test_chrome_ext_eventpage_warning.html]
 [test_chrome_ext_contentscript_unrecognizedprop_warning.html]

toolkit/components/extensions/test/mochitest/test_chrome_ext_downloads_misc.html

@@ -624,6 +624,15 @@ add_task(function* test_removal_of_incomplete_download() {
           current: true,
         },
       },
+    }, {
+      type: "onChanged",
+      data: {
+        id,
+        error: {
+          previous: null,
+          current: "USER_CANCELED",
+        },
+      },
     }]);
   is(msg.status, "success", "got onChanged event corresponding to pause");