new_contents: #!/usr/bin/env python
  import argparse
  from os import listdir
  from subprocess import run, PIPE
  import os.path
  
  parser = argparse.ArgumentParser(description='Run tests for all lessons')
  parser.add_argument('-v', dest='verbose', action='store_true', default=False, help='verbose mode')
  args = parser.parse_args()
  
  root_content = listdir('.')
  tests = list()
  for f in root_content:
      if f.startswith('.'):
          continue
  
      if not os.path.isdir(f):
          continue
  
      dir_content = listdir(f)
      py = None
      for item in dir_content:
          if item.endswith('.py'):
              py = os.path.join(f, item)
              break
  
      tests.append(py)
  
  err = 0
  for script in tests:
      ret = run(['python', script], stdout=PIPE, stderr=PIPE, universal_newlines=True)
  
      if ret.returncode or args.verbose:
          err += ret.returncode
          print('Tests for "{}" failed with {}.'.format(script, ret.returncode))
          print('STDOUT: {}'.format(ret.stdout))
          print('STDERR: {}'.format(ret.stderr))
  
  exit(err)
  
subject: Add run_tests.py. Requires python 3.5+