今日はテストを受けに学校に行ってきましたって英語でなんて言うの？

We can use 'take' or 'sit' when referring to doing an exam= both of these are appropriate in this context.
We can also use 'I had an exam at school'.

If someone has asked you what you've done today, and the answer is an exam at school, then this is the most direct way of conveying that.

"I had a test today at school." is a statement explaining what your day consisted of. So this could answer someone's question if they asked what you did today.

"I had an exam at school today" is another way to phrase to explain that you had a test. The word "exam" is short for "examination" which is another word you could use along with "test".

Both "test" and "exam" are things that you participated in answering questions on a certain topic or subject.

In common usage, you can put the word "today" at the beginning or the end of a sentence.

In my second answer, I used the phrase "most of my day was taken up with". This is not to be taken literally. Instead, this is meant to explain that the most important and noteworthy thing you did was to take an exam.

When talking about tests as a student, it's not necessary to mention school as it is implied. You can be specific about which exam or just say that you had one.

You can either 'have' an exam or 'take' an exam/test.

An exam is usually longer than a test, which is generally shorter and in class.