You should also be checking that the coordinate was added correctly, sorry I forgot to say that before, but it is very important! I didn't explain myself very well, sorry for confusion.

We shouldn't really be testing for errors like this, we should be testing the result of the code. When the code as you called it is functioning correctly no error will be raised, which means you don't need to allow for that in the test. The test should aim to capture changes in behaviour (i.e. the resulting coordinate) rather than an error, so an error shouldn't result in a test fail. If the code somehow gets broken so that this usage becomes an error then we will still be notified, but the test suite will report an error, rather than a fail, and this is OK.