a good test case

[Maybo]

At the back of the door, she pointed up. Something had been carved into the lintel, higher than Dora could reach. It was [Mr. Bowditch's] initials: AB. His original initials.
I had an idea then, one that sprang from my inability to say the word nickname. I pointed to the initials and said, "That's..." Awesome sauce was in my mind, the stupidest slang term I could think of, but a good test case.
I couldn't get it out of my mouth.

Source: Fairy Tale by Stephen King

What does "a good test case" mean?
Dora is from another world. She doesn't know human language very well. Mr. Bowditch might have taught her some words maybe.

 

[Glizdka]

A test case is an opportunity, or intentionally devised situation, in which you verify whether something works as intended.

The first attempted assassination of Donald Trump was a great test case for whether the Secret Service can effectively protect the highest ranking politicians. A test they, in the opinion of many, failed miserably, and as a result they had to revise their tactics and behavior. During the second attempted assassination, they acted much more decisively and may have saved Trump's life because of exactly that.

Studying a foreign language is all fine and dandy, but moving to another country or, in this case, another world, can provide you with many test cases for whether you can actually speak the language.