Make tmpdir resiliant to user ids that do not exist

Previously, the tmpdir fixture would fail if the current process's user
id does not correspond to a valid user (e.g. running pytest in a Docker
container with 'docker run -u').

_pytest/tmpdir.py

@@ -78,7 +78,7 @@ def get_user():
     import getpass
     try:
         return getpass.getuser()
-    except ImportError:
+    except (ImportError, KeyError):
         return None
 
 # backward compatibility