Migrate test harness off deprecated XPCOM interfaces

[1ec5]

The in-application test harness uses a variety of XPCOM interfaces that have been removed:

• nsIAtomService, which was removed for bug 1,393,692, to format table cells:
  

  avim/content/test/tester.js

  Lines 272 to 274 in 9a84083

  	const iface = Components.interfaces.nsIAtomService;
  	let atomService = Components.classes["@mozilla.org/atom-service;1"]
  	.getService(iface);

• nsIFilePicker.show(), removed in favor of nsIFilePicker.open():
  

  avim/content/test/tester.js

  Lines 75 to 81 in 9a84083

  	// https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsIFilePicker
  	let fp = Cc["@mozilla.org/filepicker;1"]
  	.createInstance(Ci.nsIFilePicker);
  	fp.init(window, "Choose Input File", Ci.nsIFilePicker.modeOpenMultiple);
  	fp.appendFilters(Ci.nsIFilePicker.filterText);
  	let rv = fp.show();

• nsILocalFile, which was removed in favor of nsIFile:
  

  avim/content/test/tester.js

  Line 90 in 9a84083

  let file = files.getNext().QueryInterface(Ci.nsILocalFile);