This is the 17437th most frequent Russian word.
тестом
"Тестом" is the instrumental case of "тесто," meaning "by/with dough" in English.
In this sentence, 'тестом' refers to 'dough', and it is the object of the action 'working with'.
Я работаю с тестом, чтобы приготовить пирог.
I am working with the dough to prepare a pie.
Here, 'тестом' refers to a 'test', meaning an examination or evaluation, used as a tool.
Мы проверим ваши знания с тестом на грамматику.
We will check your knowledge with a grammar test.
In this case, 'тестом' again refers to 'dough' that was applied to the sheet for experimental purposes.