refactor the check of executable for unit tests

pandoc was checked directly with a `os.system` call, and the output with version
and copyright of pandoc was displayed when running tests.

- replace the pandoc check with the `skipIfNoExecutable` function.
- in `skipIfNoExecutable`, the `valid_exit_code` is not needed, the executable
  is not found if an `OSError` exception is catched.

tests/support.py

@@ -114,7 +114,6 @@ def mute(returns_output=False):
     return decorator
 
 
-
 def get_article(title, slug, content, lang, extra_metadata=None):
     metadata = {'slug': slug, 'title': title, 'lang': lang}
     if extra_metadata is not None:
@@ -122,19 +121,20 @@ def get_article(title, slug, content, lang, extra_metadata=None):
     return Article(content, metadata=metadata)
 
 
-def skipIfNoExecutable(executable, valid_exit_code=1):
-    """Tries to run an executable to make sure it's in the path, Skips the tests
-    if not found.
+def skipIfNoExecutable(executable):
+    """Skip test if `executable` is not found
+
+    Tries to run `executable` with subprocess to make sure it's in the path,
+    and skips the tests if not found (if subprocess raises a `OSError`).
     """
 
-    # calling with no params the command should exit with 1
     with open(os.devnull, 'w') as fnull:
         try:
             res = subprocess.call(executable, stdout=fnull, stderr=fnull)
         except OSError:
             res = None
 
-    if res != valid_exit_code:
-        return unittest.skip('{0} compiler not found'.format(executable))
+    if res is None:
+        return unittest.skip('{0} executable not found'.format(executable))
 
     return lambda func: func