fix(pipeline): use OsString in tests

Signed-off-by: Charlotte Meyer <dev@buffet.sh>

crates/oyster_runtime/tests/it/pipeline.rs

@@ -1,10 +1,10 @@
 use std::{
     borrow::Cow,
     env,
-    ffi::OsStr,
+    ffi::{OsStr, OsString},
     fs::File,
     io::{BufRead, BufReader, Write},
-    os::unix::io::FromRawFd,
+    os::unix::{ffi::OsStringExt, io::FromRawFd},
     process,
 };
 
@@ -30,7 +30,7 @@ fn test_builtin(_: &mut Shell, _: &[Cow<OsStr>]) {
 
 /// Forks to redirect stdin, stderr, stdout, then run the commands.
 /// Relies on inserting a NUL byte in the end, so there shouldn't be NUL in the output.
-fn collect_output<F>(mut f: F) -> String
+fn collect_output<F>(mut f: F) -> OsString
 where
     F: FnMut(),
 {
@@ -65,9 +65,7 @@ where
     let mut buf = vec![];
     r.read_until(0, &mut buf).unwrap();
 
-    std::str::from_utf8(&buf[..buf.len() - 1])
-        .unwrap()
-        .to_owned()
+    OsString::from_vec(buf[..buf.len() - 1].to_vec())
 }
 
 #[test]