History, more test coverage improvements, and refactorings. (#3217)

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.

src/main.rs

@@ -64,6 +64,13 @@ fn main() -> Result<(), Box<dyn Error>> {
                 .multiple(false)
                 .takes_value(false),
         )
+        .arg(
+            Arg::with_name("no-history")
+                .hidden(true)
+                .long("no-history")
+                .multiple(false)
+                .takes_value(false),
+        )
         .arg(
             Arg::with_name("script")
                 .help("the nu script to run")
@@ -95,6 +102,7 @@ fn main() -> Result<(), Box<dyn Error>> {
         .value_of("config-file")
         .map(std::ffi::OsString::from);
     options.stdin = matches.is_present("stdin");
+    options.save_history = !matches.is_present("no-history");
 
     let loglevel = match matches.value_of("loglevel") {
         None => LevelFilter::Warn,