Bug 1617406 - Use a real image in image insertion test. r=mkmelin

--HG--
extra : rebase_source : c9f949ac830f96e0390cc06f3dcd9be3583f248e
extra : amend_source : 1a9dd73c4a99112b0a956a27db5bb61cb6d03f52

mail/test/browser/composition/browser_imageInsertionDialog.js

@@ -34,7 +34,8 @@ add_task(function test_image_insertion_dialog_persist() {
     let srcloc = mwc.window.document.getElementById("srcInput");
     srcloc.focus();
 
-    input_value(mwc, "whateverItDoesntMatterAnyway.png");
+    let file = new FileUtils.File(getTestFilePath("data/tb-logo.png"));
+    input_value(mwc, Services.io.newFileURI(file).spec);
     mwc.sleep(0);
 
     // Don't add alternate text