TOX: Add NIPY_EXTRA_TESTS to pass_env

Author: effigies

tox.ini

@@ -51,12 +51,14 @@ install_command =
     {opts} {packages}
 pip_pre =
   pre: true
-# getpass.getuser() sources for Windows:
 pass_env =
+  # getpass.getuser() sources for Windows:
   LOGNAME
   USER
   LNAME
   USERNAME
+  # Environment variables we check for
+  NIPY_EXTRA_TESTS
 extras = test
 deps =
   # NEP29/SPEC0 + 1yr: Test on minor release series within the last 3 years