Fix slicing in tests, although not sure it ever has enough workers #3480

Job	Run time
devcontainer / build (amd64, 12.0, conda)	42m 36s
devcontainer / build (amd64, 12.0, pip)	43m 23s
checks / other-checks	39s
checks / check-style	1m 5s
wheel-build-pylibraft / compute-matrix	1s
conda-cpp-build / compute-matrix	3s
wheel-build-pylibraft / 11.8.0, 3.9, amd64, centos7	25m 40s
wheel-build-pylibraft / 11.8.0, 3.10, amd64, centos7	25m 52s
wheel-build-pylibraft / 11.8.0, 3.9, arm64, rockylinux8	25m 41s
wheel-build-pylibraft / 11.8.0, 3.10, arm64, rockylinux8	25m 48s
wheel-build-pylibraft / 12.0.1, 3.9, amd64, centos7	25m 39s
wheel-build-pylibraft / 12.0.1, 3.10, amd64, centos7	25m 40s
wheel-build-pylibraft / 12.0.1, 3.9, arm64, rockylinux8	25m 35s
wheel-build-pylibraft / 12.0.1, 3.10, arm64, rockylinux8	25m 27s
conda-cpp-build / build (11.8.0, ubuntu22.04, amd64, 3.10)	42m 38s
conda-cpp-build / build (11.8.0, ubuntu22.04, arm64, 3.10)	43m 15s
conda-cpp-build / build (12.0.1, ubuntu22.04, amd64, 3.10)	27m 55s
conda-cpp-build / build (12.0.1, ubuntu22.04, arm64, 3.10)	26m 41s
wheel-tests-pylibraft	0s
wheel-build-raft-dask	0s
wheel-tests-raft-dask	0s
conda-cpp-checks	0s
conda-python-build	0s
conda-cpp-tests	0s
docs-build	0s
conda-python-tests	0s
pr-builder	0s
	7h 13m 38s

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