Improve import-export unittest a bit

- Increase timeouts from 10 to 30 seconds (this still breaks when the
  machine is busy, e.g. using bonnie++)
- Depend on only one timeout per test instead of three
- Reset variables before each test

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>