fix(test): Add back in newlines to diffs

Errors like this aren't too helpful
```
error: stderr did not match:
1   1     error: failed to parse manifest at `[..]`2   2     3   3     Caused by:4   4       TOML parse error at line 3, column 275   5         |6   6       3 |                 version = 17
  7         |                           ^8        -  invalid type: integer `1`, expected SemVer version    8    +  invalid type: integer `1`, expected a string or workspace
```

This was broken in #12581

crates/cargo-test-support/src/diff.rs

@@ -132,7 +132,7 @@ pub fn render_colored_changes<T: fmt::Display>(changes: &[Change<T>]) -> String
             Change::Remove(i, s) => (format!("{:<4}     ", i), '-', red, s),
             Change::Keep(x, y, s) => (format!("{:<4}{:<4} ", x, y), ' ', dim, s),
         };
-        write!(
+        writeln!(
             buffer,
             "{dim}{nums}{reset}{bold}{sign}{reset}{color}{text}{reset}"
         )