Fix assertion in dask_cudf test_struct_explode (#11170)

The current implementation of `test_accessor.py::test_struct_explode` relies on `got` having a single partition. However, an [upstream fix](dask/dask#9221) will change the behavior of `from_cudf` to return the **correct** number of partitions (2) soon.  Therefore, this PR is just a preemptive fix.

Authors:
  - Richard (Rick) Zamora (https://github.com/rjzamora)

Approvers:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

URL: #11170

python/dask_cudf/dask_cudf/tests/test_accessor.py

@@ -525,5 +525,5 @@ def test_dask_struct_field_Int_Error(data):
 def test_struct_explode(data):
     expect = Series(data).struct.explode()
     got = dgd.from_cudf(Series(data), 2).struct.explode()
-
-    assert_eq(expect, got.compute())
+    # Output index will not agree for >1 partitions
+    assert_eq(expect, got.compute().reset_index(drop=True))