fixed up paths in test_output_template so it works on windows

Author: tclose

pydra/engine/tests/test_helpers_file.py

@@ -395,7 +395,7 @@ class MyCommand(ShellCommandTask):
     task = MyCommand(in_file=filename)
     assert task.cmdline == f"my {filename}"
     task.inputs.optional = True
-    assert task.cmdline == f"my {filename} --opt {task.output_dir}/file.out"
+    assert task.cmdline == f"my {filename} --opt {task.output_dir / 'file.out'}"
     task.inputs.optional = False
     assert task.cmdline == f"my {filename}"
     task.inputs.optional = "custom-file-out.txt"