Do I need a return ticket or is a one-way ticket sufficient for Test and Go in Thailand?

No

Return

Stock answer as asked many times here.

If you are entering on a long term visa, like a Non O or Non OA or Thailand Elite you do not need an onward flight out from Thailand.

If on a visa exempt entry or tourist visa you need to have a flight ticket out of Thailand within the amount of days that your initial entry allows, regardless that you can get an extension in country. It doesn’t need to be a return ticket, any ticket out of Thailand will suffice.

In theory immigration can deny you entry if you don’t but they rarely ever would, because they know you can get an extension.

Almost always the hang up will be when you try to board your plane. Many airlines will not allow you to board without an onward ticket within the number of days.

Some will let you. Many won’t. You probably don’t want to be the one scrambling to buy an onward ticket at the check in desk. Best option is a changeable ticket, and just change the date when you’re here.

Alternatively you can Google “onward flight ticket” and you’ll find a plethora of companies willing to sell you one for around $10. A real ticket, Phuket to Singapore, would be approx £25/$35 depending on the time of year.

You’ll probably get a number of replies saying “I managed to enter with no problem, therefore you won’t have one either.” Well you won’t find out which way it’ll go until you get to the check in desk.

Return ticket won't matter for Test & Go, but it might matter for your visa.