Improve our tests that use fixtures

[bugarela]

Ideas:

1. Format test fixture json file as suggested by @p-offtermatt in https://github.com/informalsystems/quint/pull/1421#pullrequestreview-1985582480, this would make the diffs much easier to read (currently impossible to read)
2. Use a proper snapshot lib to handle these.

[bugarela]

We could take advantage of jd, which shows formatted json difference:

git show main:quint/testFixture/_1016nonConstOverride.json | jd testFixture/_1016nonConstOverride.json
@ ["table","8","id"]
- 6
+ 2

But we should first check if there are any snapshot tools - I want to avoid having to maintain the tool in house, even if its simple.