Test support fixes (#4517)

* Fix failing pipeline()

The `skip(1)` was there likely to remove the welcome message.

* Fix typo

* Fix nu! test macro to enter cwd correctly

Nushell's current working directory is determined primarily by the PWD
environment variable.

crates/nu-test-support/src/lib.rs

@@ -26,8 +26,8 @@ impl Outcome {
 
 pub fn pipeline(commands: &str) -> String {
     commands
+        .trim()
         .lines()
-        .skip(1)
         .map(|line| line.trim())
         .collect::<Vec<&str>>()
         .join(" ")