Add known external tests (#5216)

* Add known external tests

* Add some documentation to the tests

* Document test_hello example

* Set PWD in run_test

examples/test_hello.rs

@@ -0,0 +1,6 @@
+/// This function is only meant to be used as part of the test suite
+/// as a simple, cross-platform executable with known output.
+
+fn main() {
+    println!("test-hello");
+}

src/tests.rs

@@ -5,6 +5,7 @@ mod test_engine;
 mod test_env;
 mod test_hiding;
 mod test_iteration;
+mod test_known_external;
 mod test_math;
 mod test_modules;
 mod test_parser;
@@ -42,6 +43,10 @@ pub fn run_test(input: &str, expected: &str) -> TestResult {
 
     let mut cmd = Command::cargo_bin("nu")?;
     cmd.arg(name);
+    cmd.env(
+        "PWD",
+        std::env::current_dir().expect("Can't get current dir"),
+    );
 
     writeln!(file, "{}", input)?;
 

src/tests/test_known_external.rs

@@ -0,0 +1,35 @@
+use crate::tests::{fail_test, run_test, TestResult};
+
+#[test]
+fn known_external_runs() -> TestResult {
+    run_test(
+        r#"extern "cargo run" [-q, --example: string, ...args]; cargo run -q --example test_hello"#,
+        "test-hello",
+    )
+}
+
+#[test]
+fn known_external_unknown_flag() -> TestResult {
+    fail_test(
+        r#"extern "cargo run" [-q, --example: string, ...args]; cargo run -d"#,
+        "command doesn't have flag",
+    )
+}
+
+/// GitHub issues #5179, #4618
+#[test]
+fn known_external_alias() -> TestResult {
+    run_test(
+        r#"extern "cargo run" [-q, --example: string, ...args]; alias cr = cargo run; cr -q --example test_hello"#,
+        "test-hello",
+    )
+}
+
+/// GitHub issues #5179, #4618
+#[test]
+fn known_external_subcommand_alias() -> TestResult {
+    run_test(
+        r#"extern "cargo run" [-q, --example: string, ...args]; alias c = cargo; c run -q --example test_hello"#,
+        "test-hello",
+    )
+}