Fix groupWithin test that intermittently fails

[mpilquist]

https://github.com/typelevel/fs2/runs/2746360895?check_suite_focus=true#step:6:849

==> X fs2.StreamCombinatorsSuite.groupWithin - accumulation and splitting  0.491s munit.ComparisonFailException: /home/runner/work/fs2/fs2/core/shared/src/test/scala/fs2/StreamCombinatorsSuite.scala:852
850
851:        .toList
851
852:        .assertEquals(expected)
852
853:    }
853
values are not the same
854
=> Obtained
855
List(
856
  List(
857
    1,
858
    2,
859
    3
860
  ),
861
  List(
862
    4,
863
    5,
864
    6,
865
    7
866
  ),
867
  List(
868
    8,
869
    9,
870
    10,
871
    11,
872
    12
873
  ),
874
  List(
875
    13,
876
    14,
877
    15,
878
    16,
879
    17
880
  ),
881
  List(
882
    18,
883
    19,
884
    20,
885
    21,
886
    22
887
  )
888
)
889
=> Diff (- obtained, + expected)
890
     6,
891
-    7
892
+    7,
893
+    8
894
   ),
895
   List(
896
-    8,
897
     9,
898
     11,
899
-    12
900
+    12,
901
+    13
902
   ),
903
   List(
904
-    13,
905
     14,
906
     16,
907
-    17
908
+    17,
909
+    18
910
   ),
911
   List(
912
-    18,
913
     19,

[Angel-O]

I might have a fix for it 🙏🏾