I wouldn't worry too much.... these guys had been in Thailand for 2 years.... they had a decent run... I mean, every country has it's immigration laws, I'm not sure about the US but it's actually quite hard for a thai to get in to the UK. Dress smart, have the 3 Proof ofs. Be polite, have a good story
It may be that your carrier will want to see an onward ticket within the initial 30 day period.... if so you can just buy a "rental" ticket quite cheaply on line
Test and go has nothing to do with being denied entry.. If the IO thinks you're up to no good in thailand OR working illegally he MAY deny you entry.... make sure you have the three proof ofs....that is... 20,000thb in cash, onward ticket and proof of accommodation. Best of luck