Implement strict attribute for challenges.

This way is possible to toggle equality check for empty lines.

bin/docker_cmd.py

@@ -45,6 +45,13 @@ def __exit__(self, type, value, traceback):
         signal.alarm(0)
 
 
+def strip_output(output, challenge, script_name, return_token):
+    strict = 'expected_output' in challenge and 'strict' in challenge['expected_output']
+    if not strict: output = re.sub(r'\n{2,}', '\n', output).rstrip()
+    output = re.sub(r'{}\d+'.format(return_token), '', output).rstrip()
+    output = re.sub(r'/tmp/.{}: line \d+: (.*)'.format(script_name), r'\1', output)
+    return output
+
 def output_from_cmd(cmd, challenge, docker_version=None, docker_base_url=None, tls_settings=None):
     if tls_settings:
         tls_config = docker.tls.TLSConfig(**tls_settings)
@@ -92,8 +99,7 @@ def output_from_cmd(cmd, challenge, docker_version=None, docker_base_url=None, t
             if return_code_match is None:
                 raise DockerValidationError("Unable to determine return code from command")
             return_code = int(return_code_match.group(1))
-            output = re.sub(r'{}\d+'.format(return_token), '', output).rstrip()
-            output = re.sub(r'/tmp/.{}: line \d+: (.*)'.format(script_name), r'\1', output)
+            output = strip_output(output, challenge, script_name, return_token)
             if return_code == 124:
                 output += "\n** Command timed out after {} seconds **".format(CMD_TIMEOUT)
             if 'tests' in challenge:

challenges.yaml

@@ -10,6 +10,7 @@
 #   tests: tests to run after the command completes
 #   version: if the challenge is modified this number should be bumped
 #            refresh the cache.
+#   strict: empty lines must not be ignored
 #   author: Add a field for contributions.
   - slug: hello_world
     version: 3
@@ -419,6 +420,7 @@
         - ''
         - '# **************'
         - '# Reverse the README'
+      strict: true
   - slug: remove_duplicate_lines
     version: 3
     author: cmdchallenge