Playground infraestructure (tests, etc) additions. (#3179)

* 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)

crates/nu-command/tests/commands/mod.rs

@@ -1,7 +1,4 @@
 mod append;
-mod autoenv;
-mod autoenv_trust;
-mod autoenv_untrust;
 mod cal;
 mod cd;
 mod compact;