mount_test: for invalidate tests, use second config to avoid flake
mount_test: for invalidate tests, use second config to avoid flake

There were two mountpoints, but they shared the same config, so
invalidations weren't being sent to the kernel side of the second
mountpoint.

Issue: KBFS-3577

keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client