[bugfix] Ensure the /tmp directory is used by the tests

On OS X, another one was used, that can't be mounted inside a Docker container

beat/core/test/__init__.py

@@ -35,7 +35,7 @@ import pkg_resources
 
 
 prefix = pkg_resources.resource_filename(__name__, 'prefix')
-tmp_prefix = tempfile.mkdtemp(prefix=__name__, suffix='.tmpdir')
+tmp_prefix = tempfile.mkdtemp(prefix=__name__, suffix='.tmpdir', dir='/tmp')
 
 
 def teardown_package():