testing purpose or testing purposes?

80% of the data was selected to build the rules, and the remaining was used for a testing purpose.
OR
80% of the data was selected to build the rules, and the remaining was used for testing purposes.

Note: The remaining data was to test all rules.

(Not a Teacher)

Though I wouldn't say that the first sentence is wrong, it does sound weird. If you want to keep 'purpose' singular, I would write 'the purpose of testing' to make this sentence sound better.
The second sentence sounds the most natural.

Also, I would replace 'remaining' with 'remainder'.

Do you mean that I should replace remaining data by remainder? Thanks

(Not a Teacher)

You could use 'the remaining data', too, but I feel that 'the remainder' flows better in this sentence.