さっきのテスト解くのに思ったより時間がかかってしまったよって英語でなんて言うの？

As it is implied you are talking about test the pronoun ‘that ‘can be used to talk about it. That took longer or the test took longer both acceptable. Similar with it in third example. Test is implied so you can use pronoun ‘it’ without an antecedent

When you start a task, project, or in this case, a test, you may not have any real idea about how long it will take to complete. However it is in human nature to make a prediction, guess or anticipate how long it will take. ' I anticipated it would take me an hour to walk home, but it actually took me two hours.'

*The test took longer than I expected. -People have different expectations. Sometimes reality doesn't meet our expectations. Example Sentences: I'm disappointed, I expected it would rain today. I expect to get 95% in Maths.

The first answer is a simple way of saying this. If you wanted to use more elaborate English, you could use the second answer. The verb 'underestimated' when used in past tense means you made a guess or an estimate about something but your guess was not enough. You made a guess that the test would take longer than it did.

When you expect or predict something to happen then this is called 'anticipate' you can also say 'I didn't expect' or 'than i thought' these all mean you didn't think something would happen the way it did

This is a way to tell someone that the time period they had in mind for the test was actually longer than they thought it would be.