Add basic spec for getAccentColor

spec/api-system-preferences-spec.js

@@ -3,11 +3,23 @@ const {remote} = require('electron')
 const {systemPreferences} = remote
 
 describe('systemPreferences module', function () {
-  if (process.platform !== 'darwin') {
-    return
-  }
+  describe('systemPreferences.getAccentColor', function () {
+    if (process.platform !== 'win32') {
+      return
+    }
+
+    it('should return a non-empty string', function () {
+      let accentColor = systemPreferences.getAccentColor();
+      assert.notEqual(accentColor, null);
+      assert(accentColor.length > 0);
+    })
+  })
 
   describe('systemPreferences.getUserDefault(key, type)', function () {
+    if (process.platform !== 'darwin') {
+      return
+    }
+
     it('returns values for known user defaults', function () {
       let locale = systemPreferences.getUserDefault('AppleLocale', 'string')
       assert.notEqual(locale, null)