Not sure what writing code to the tests actually means, is that even correct English (I'm not a native English speaker)?

It’s idiomatic, and it usually has a negative connotation. An analogous example is criticising a school for “teaching to the exam”, usually implying that the school’s teaching is inappropriately prioritising getting the student a good grade in their exam, at the the expense of giving the student a good general education in the subject.

Here’s a contrived example of writing code to the tests, in the negative sense. Given this test:

    def test_add():
        assert(add(1, 1) == 2)

we could write this obviously broken implementation of the add function, which is nevertheless the simplest code that passes the test:

    def add(a, b):
        return 2