tests: Fix 'order_start' on BundleReorderTest unit test

tests: Fix 'order_start' on BundleReorderTest unit test

The 'order_start' parameter on the reorder form should be a patch ID,
not the 'order' field of BundlePatch.

This fixes the following test failure:

> ======================================================================
> FAIL: testBundleReorder (patchwork.tests.bundles.BundleReorderTest)
> ----------------------------------------------------------------------
> Traceback (most recent call last):
>   File "/mnt/common/code/admin/rh-patchwork/apps/patchwork/tests/bundles.py", line 358, in testBundleReorder
>     self.failUnlessEqual(response.status_code, 200)
> AssertionError: 404 != 200
>
> ----------------------------------------------------------------------
> Ran 123 tests in 4.527s
>
> FAILED (failures=1)

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>