Shot in the dark here as I haven’t used windows since XP 
, but you may have to use the set or setx command to specify ENV vars.
C:\path\to\project> setx TEST_WATCH=1
Success: Specified value was saved.
C:\path\to\project> meteor test --driver-package meteortesting:mocha
Some things I have read say you may need to restart your command prompt after running setx.