Human-readable configuration output

[sunshowers]

Related:

• [meta] tell users what configuration their tests are running with #1525
• Human-readable configuration output #1526

Users should have a way to tell what settings their tests are running with, and what the overall nextest configuration is.

We have some basic support within cargo nextest show-config, and this issue is to expand and complete that.

Some things to think about with human-readable configuration output:

• Is show-config enough for common use cases? Do we want to show per-test and per-binary settings in nextest list as well, as an optional thing to include?
• What's the best place to include this information? For settings by test, nextest list is a good option, as is. For settings by
• There are a lot of config options that are primarily "group by" operations. Does it make sense to build out some general code for this? Probably easiest to tell after the first 2-3.
• How similar do we want group-by operations to feel? Subcommands or arguments? (Having subcommands gives us an easy way to specify particular options, but feels a bit more heavyweight. Worth thinking about.)
• We should also tell users where a particular bit of configuration is coming from. I believe we track the necessary information already, but this should be double-checked.

[sunshowers]

I don't currently have the time to work on this, but would be happy to work with you to start adding this if you're interested.