test(doctest): add doctest for megengine dir

GitOrigin-RevId: c31bbb1afc0003cb3700ecdde7db6dafc6c5f46e

imperative/python/test/run.sh

 #!/bin/bash -e
 
-test_dirs="test"
+test_dirs="test megengine"
 TEST_PLAT=$1
 
 if [[ "$TEST_PLAT" == cpu ]]; then