Ledger: Fix tests so that they pass regardless of system locale

[sanderploegsma]

I tried running the tests for the ledger exercise on my local machine and they failed as well. I suspect it has to do with the locale of the machine the tests are running on. The test runner probably uses the en-US locale by default, and so the exercise and tests have been implemented based on that assumption. My machine's locale is set to nl-NL, I can imagine that you also have a non-US locale set?

This would mean that students attempting to solve this exercise locally may run into the same issue, depending on their locale settings. That sounds like a bug in the exercise, so perhaps it should be looked into in a new issue.

Originally posted by @sanderploegsma in #2627 (comment)