spec: Add a simple test case for desktopCapturer

spec/api-desktop-capturer.coffee

@@ -0,0 +1,9 @@
+assert = require 'assert'
+{desktopCapturer} = require 'electron'
+
+describe 'desktopCapturer', ->
+  it 'should returns something', (done) ->
+    desktopCapturer.getSources {types: ['window', 'screen']}, (error, sources) ->
+      assert.equal error, null
+      assert.notEqual sources.length, 0
+      done()