Improvements overall to Nu. Also among the changes here, we can also be more confident towards incorporating `3041`. End to end tests for checking envs properly exported to externals is not added here (since it's in the other PR)
A few things added in this PR (probably forgetting some too)
* no writes happen to history during test runs.
* environment syncing end to end coverage added.
* clean up / refactorings few areas.
* testing API for finer control (can write tests passing more than one pipeline)
* can pass environment variables in tests that nu will inherit when running.
* No longer needed.
* no longer under a module. No need to use super.
* Playground infraestructure (tests, etc) additions.
A few things to note:
* Nu can be started with a custom configuration file (`nu --config-file /path/to/sample_config.toml`). Useful for mocking the configuration on test runs.
* When given a custom configuration file Nu will save any changes to the file supplied appropiately.
* The `$nu.config-path` variable either shows the default configuration file (or the custom one, if given)
* We can now run end to end tests with finer grained control (currently, since this is baseline work, standard out) This will allow to check things like exit status, assert the contents with a format, etc)
* Remove (for another PR)
The autoenv logic mutates environment variables in the running session as
it operates and decides what to do for trusted directories containing `.nu-env`
files. Few of the ways to interact with it were all in a single test function.
We separate out all the ways that were done in the single test function to document
it better. This will greatly help once we start refactoring our way out from setting
environment variables this way to just setting them to `Scope`.
This is part of an on-going effort to keep variables (`PATH` and `ENV`)
in our `Scope` and rely on it for everything related to variables.
We expect to move away from setting (`std::*`) envrironment variables in the current
running process. This is non-trivial since we need to handle cases from vars
coming in from the outside world, prioritize, and also compare to the ones
we have both stored in memory and in configuration files.
Also to send out our in-memory (in `Scope`) variables properly to external
programs once we no longer rely on `std::env` vars from the running process.
