message: Add tests/ProjectBuilder::at
diff: @@ -126,6 +126,12 @@ impl ProjectBuilder {
  }
  }
  + #[allow(dead_code)]
  + pub fn at<P: AsRef<Path>>(mut self, path: P) -> Self {
  + self.root = Project::Rooted(paths::root().join(path));
  + self
  + }
  +
  /// Add a file to the project.
  pub fn file<B: AsRef<Path>>(mut self, path: B, body: &str) -> Self {
  self._file(path.as_ref(), body);
  