More extensive test suite for std.format

It's by far the most complex stdlib function.

We had two bugs already and we are considering a native implementation in go-jsonnet. We already have quite a lot of tests, but we can still do better.

We can even auto-generate a lot of tests and generate the correct results with Python (we're supposed to be mostly compatible).

Here's what we have currently: https://github.com/google/jsonnet/blob/v0.17.0/test_suite/format.jsonnet