This is a simple computation over the JSON structure of the schema,
so it doesn’t need pulling in another programming language and a
dependency on an external C program to interpret it.
Extracts the `"default"` values from the schema and creates a synthetic
TOML file holding all the defaults according to the schema. This is done
through some `jq` magic and is not perfect but rather a best effort.
If `jq` is not available, the test is skipped; in CI `jq` is required.
The test then run `jj config get` in the test env for each key in that
defaults file and compares the resulting value with the schema default.
For a few keys, there are actually no defaults known to `jj config get`,
because they are hard-coded or dynamic. These exceptions are intercepted
and explained in the test.
