new_contents: import subprocess
  import sys
  from time import sleep
  
  import os
  
  
  def filter_stacktrace(s):
      ret = ""
      for line in s.splitlines(keepends=True):
          if "_user$project$" in line:
              ret += line.replace("_user$project$", "").replace("$", ".", 1)
      return ret
  
  try:  # windows
      x = subprocess.run([os.environ['USERPROFILE'] + r"\AppData\Roaming\npm\elm-test.cmd", "--stack-trace-limit=300"], stdout=subprocess.PIPE,
                         stderr=subprocess.PIPE)
  except:  # macos / linux
      x = subprocess.run([r"/usr/local/bin/elm-test", "--stack-trace-limit=300"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
  
  print("#stdout#")
  sys.stdout.write(x.stdout.decode("utf-8"))
  print("#stdout#")
  sleep(0.2)
  print("#!stderr!#")
  stderr = x.stderr.decode("utf-8")
  sys.stderr.write(filter_stacktrace(stderr))
  print("#!stderr!#")
  exit(x.returncode)
  
subject: Add python elm-test test runner.