testing: add a `write_file` function

The function is kinda trivial, but having a simple function avoid silly mistake.

Differential Revision: https://phab.mercurial-scm.org/D10477

mercurial/testing/__init__.py

@@ -1,30 +1,35 b''
 from __future__ import (
     absolute_import,
     division,
 )
 
 import os
 import time
 
 
 # work around check-code complains
 #
 # This is a simple log level module doing simple test related work, we can't
 # import more things, and we do not need it.
 environ = getattr(os, 'environ')
 
 
 def _timeout_factor():
     """return the current modification to timeout"""
     default = int(environ.get('HGTEST_TIMEOUT_DEFAULT', 1))
     current = int(environ.get('HGTEST_TIMEOUT', default))
     return current / float(default)
 
 
 def wait_file(path, timeout=10):
     timeout *= _timeout_factor()
     start = time.time()
     while not os.path.exists(path):
         if time.time() - start > timeout:
             raise RuntimeError(b"timed out waiting for file: %s" % path)
         time.sleep(0.01)
+
+
+def write_file(path, content=b''):
+    with open(path, 'wb') as f:
+        f.write(content)