Fix a bug where stdout was copying stderr

In the migration of the Ruby buildpack to use this crate I noticed a regression:

```
  - Running `bash -c "ps aux | grep cargo"`

            rschneeman       26398   0.0  0.0 408417520   9600 s004  S+   11:24AM   0:00.13 /Users/rschneeman/.cargo/bin/cargo-watch watch -c -x ltest
      rschneeman       17561   0.0  0.0 408494848    848 s000  R+    2:39PM   0:00.00 grep cargo
      rschneeman       17559   0.0  0.0 408627920   3088 s000  S+    2:39PM   0:00.00 bash -c ps aux | grep cargo
      rschneeman       15128   0.0  0.0 408433904  10176 s006  S+    2:35PM   0:00.40 /Users/rschneeman/.cargo/bin/cargo-watch watch -c -x ltest

  - Done (< 0.1s)
```

I isolated it to the `fun_run` PR heroku/buildpacks-ruby#232 and after some debugging found it was due to accidentally copying `child.stdout` for `child_stderr`. I added a test that verifies that stderr is redirected correctly.

Cargo.toml

@@ -15,3 +15,6 @@ regex = "1"
 
 [features]
 which_problem = ["dep:which_problem"]
+
+[dev-dependencies]
+pretty_assertions = "1"

src/command.rs

@@ -23,7 +23,7 @@ pub(crate) fn output_and_write_streams<OW: Write + Send, EW: Write + Send>(
  let stdout_thread = mem::take(&mut child.stdout).map(|mut child_stdout| {
  scope.spawn(move || std::io::copy(&mut child_stdout, &mut stdout))
  });
- let stderr_thread = mem::take(&mut child.stdout).map(|mut child_stderr| {
+ let stderr_thread = mem::take(&mut child.stderr).map(|mut child_stderr| {
  scope.spawn(move || std::io::copy(&mut child_stderr, &mut stderr))
  });
 
@@ -56,11 +56,12 @@ pub(crate) fn output_and_write_streams<OW: Write + Send, EW: Write + Send>(
 #[cfg(test)]
 mod test {
  use super::*;
+ use pretty_assertions::assert_str_eq;
  use std::process::Command;
 
  #[test]
  #[cfg(unix)]
- fn test_output_and_write_streams() {
+ fn test_output_and_write_streams_stdout() {
  let mut stdout_buf = Vec::new();
  let mut stderr_buf = Vec::new();
 
@@ -76,6 +77,20 @@ mod test {
  assert_eq!(output.stdout, "Hello World!".as_bytes());
  assert_eq!(output.stderr, Vec::<u8>::new());
  }
+
+ #[test]
+ #[cfg(unix)]
+ fn test_output_and_write_streams_stderr() {
+ let mut stdout_buf = Vec::new();
+ let mut stderr_buf = Vec::new();
+
+ let mut cmd = Command::new("bash");
+ cmd.args(["-c", "echo -n Hello World! >&2"]);
+
+ let _ = output_and_write_streams(&mut cmd, &mut stdout_buf, &mut stderr_buf).unwrap();
+
+ assert_str_eq!(&String::from_utf8_lossy(&stderr_buf), "Hello World!");
+ }
 }
 
 /// Constructs a writer that writes to two other writers. Similar to the UNIX `tee` command.