backend: fix default flag value in test stub (#2551)
backend: fix default flag value in test stub (#2551)

Summary:
This doesn’t actually end up mattering, but the default value for each
flag should be a valid member of that flag’s domain. This is a hold-over
from a previous formulation of `--generic_data`.

Test Plan:
That tests pass suffices.

wchargin-branch: fix-test-default-flag
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard