More extensive test suite for std.format

[sbarzowski]

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

[sbarzowski]

We can also consider writing some property tests. E.g. checking that for all x, std.abs(std.parseJson(std.format('%.3f, [x])) - x) < 0.001.

[szeiger]

We also hit a format bug in sjsonnet: databricks/sjsonnet#144. The current implementation in jsonnet also doesn't match Python's formatting (which is linked as the only reference for the formatter behavior in the spec) in this case.