test: fix temp-dir option in tools/test.py

If a temp-dir is specified and already exists, the NODE_TEST_DIR
environment variable will never be set. This fixes that.

PR-URL: nodejs#10723
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

tools/test.py

@@ -1628,9 +1628,9 @@ def Main():
 
   tempdir = os.environ.get('NODE_TEST_DIR') or options.temp_dir
   if tempdir:
+    os.environ['NODE_TEST_DIR'] = tempdir
     try:
       os.makedirs(tempdir)
-      os.environ['NODE_TEST_DIR'] = tempdir
     except OSError as exception:
       if exception.errno != errno.EEXIST:
         print "Could not create the temporary directory", options.temp_dir