[TRTLLM-17333][ci] Scope GDN decode waive to the index-slice test

The file-level waive for nvbugs/6842831 also skips
test_fi_decode_misaligned_ab_slices, which would leave the replacement
coverage for the removed 8-GPU Ray stage inactive in CI. Narrow it to
test_fi_decode_misaligned_index_slice.

Signed-off-by: Erin Ho <14718778+hchings@users.noreply.github.com>

Author: hchings

tests/integration/test_lists/waives.txt

@@ -170,7 +170,7 @@ unittest/_torch/modeling_v2/comm/test_modeling_v2_reducescatter_op_matrix.py::te
 unittest/_torch/modeling_v2/models/glm_moe_dsa/glm_5_2_nvfp4__sm_107__tp16/dsa/test_modeling_v2_hist_topk_op_matrix.py::test_hist_topk_op_matrix SKIP (https://nvbugs/7004363)
 unittest/_torch/modeling_v2/models/glm_moe_dsa/glm_5_2_nvfp4__sm_107__tp16/spec/test_modeling_v2_draft_ehproj_table.py::test_draft_ehproj_table_op_matrix SKIP (https://nvbugs/7004363)
 unittest/_torch/modeling_v2/models/glm_moe_dsa/glm_5_2_nvfp4__sm_107__tp16/spec/test_modeling_v2_draft_sample_glue.py::test_draft_sample_glue_op_matrix SKIP (https://nvbugs/7004363)
-unittest/_torch/modules/mamba/test_flashinfer_gdn_decode.py SKIP (https://nvbugs/6842831)
+unittest/_torch/modules/mamba/test_flashinfer_gdn_decode.py::test_fi_decode_misaligned_index_slice SKIP (https://nvbugs/6842831)
 unittest/_torch/modules/tests_lora_modules/test_nemotron_h_lora_sanity.py::TestNemotronHLoRA::test_lora_pp2_sanity SKIP (https://nvbugs/6428124)
 unittest/_torch/moe/test_moe_backend.py::test_moe_backend[act=Relu2-e60_k4_h2048_i1408-seq=8-dtype=torch.bfloat16-backend=TRTLLM-quant=NVFP4-routing=Renormalize] SKIP (https://nvbugs/5989912)
 unittest/_torch/moe/test_moe_backend.py::test_trtllm_bf16_unquantized_moe[deepseekv3-relu2-8-separated_routing] SKIP (https://nvbugs/6778221)