Sorry for reviving this, but if you're on a Mac, you can also send yourself an email with the desired subject emojis, and when you receive your email, just hit Command-Option-U on your keyborad and copy the subject..
This Is A Test
comes back as:
which you can just copy-paste into PHPList
watch out that not all emojis can be read by all.. best check compatibility first on a site like https://emojipedia.org
Hope this helps (sorry again for reviving)