treefmt: use structuredAttrs instead of passAsFile in test (#515389)

pkgs/by-name/tr/treefmt/tests.nix

@@ -72,10 +72,6 @@ in
     runCommand "run-nixfmt-example"
       {
         nativeBuildInputs = [ nixfmtExamplePackage ];
-        passAsFile = [
-          "input"
-          "expected"
-        ];
         input = ''
           {
             foo="bar";
@@ -90,18 +86,19 @@ in
             list = [ ];
           }
         '';
+        __structuredAttrs = true;
       }
       ''
         export XDG_CACHE_HOME=$(mktemp -d)
         # The example config assumes the tree root has a .git/index file
         mkdir .git && touch .git/index
 
-        # Copy the input file, then format it using the wrapped treefmt
-        cp $inputPath input.nix
+        # Create the input file, then format it using the wrapped treefmt
+        printf "%s" "$input" > input.nix
         treefmt
 
         # Assert that input.nix now matches expected
-        if diff -u $expectedPath input.nix; then
+        if diff -u <(printf "%s" "$expected") input.nix; then
           touch $out
         else
           echo