test: set locale for datetime-change-notify test

The time zone output by toString() will change with user's language, use toLocaleString() to set the time zone.

This test will fail on a computer with the language set to Chinese.

Example:

use toString()

new Date().toString() 
// output: Thu May 20 2021 05:18:38 GMT+0000 (协调世界时)

use toLocaleString()

new Date().toLocaleString('en-US', { timeZoneName: 'long' }); 
// output: date 5/20/2021, 5:24:25 AM Coordinated Universal Time