Add a passing test for interactivity on slow pipelines (#12865)

# Description

This PR adds a single test to assert interactivity on slow pipelines

Currently the timeout is set to 6 seconds, as the test can sometimes
take ~3secs to run on my local m1 mac air, which I don't think is an
indication of a slow pipeline, but rather slow test start up time...

tests/plugins/stream.rs

@@ -1,3 +1,5 @@
+use rstest::rstest;
+
 use nu_test_support::nu_with_plugins;
 use pretty_assertions::assert_eq;
 
@@ -190,3 +192,18 @@ fn generate_sequence() {
 
     assert_eq!(actual.out, "[0,2,4,6,8,10]");
 }
+
+#[rstest]
+#[timeout(std::time::Duration::from_secs(6))]
+fn echo_interactivity_on_slow_pipelines() {
+    // This test works by putting 0 on the upstream immediately, followed by 1 after 10 seconds.
+    // If values aren't streamed to the plugin as they become available, `example echo` won't emit
+    // anything until both 0 and 1 are available. The desired behavior is that `example echo` gets
+    // the 0 immediately, which is consumed by `first`, allowing the pipeline to terminate early.
+    let actual = nu_with_plugins!(
+        cwd: "tests/fixtures/formats",
+        plugin: ("nu_plugin_example"),
+        r#"[1] | each { |n| sleep 10sec; $n } | prepend 0 | example echo | first"#
+    );
+    assert_eq!(actual.out, "0");
+}