[BUG] Flaky cpp test `AnnIVFFlatTestF_half.AnnIVFFlat/21`

[julianmi]

AnnIVFFlat test configuration 21 with { 1000, 10000, 2051, 16, 40, 1024, distance::InnerProduct, 1,0,0} shows very low recall. This test becomes flaky depending on the architecture used and tests start to fail with recall values below the threshold. We believe this is mathematical property of the test configuration and propose to skip this test until we find a better parameter set.

[ RUN      ] AnnIVFFlatTest/AnnIVFFlatTestF_half.AnnIVFFlat/21
[   331][14:12:46:144510][info  ] Recall = 0.030875 (494/16000), the error is 0.9% above the threshold (eps = 0.005000).
/home/coder/cuvs/cpp/tests/neighbors/ann_ivf_flat/../ann_ivf_flat.cuh:260: Failure
Value of: eval_neighbours(indices_naive, indices_ivfflat, distances_naive, distances_ivfflat, ps.num_queries, ps.k, eps, min_recall)
  Actual: false (actual recall (0.030875) is lower than the minimum expected recall (0.0390625); eps = 0.004999999888241291. )
Expected: true

[   331][14:12:47:605797][info  ] Recall = 0.071750 (1148/16000), the error is 3.4% below the threshold (eps = 0.005000).
[  FAILED  ] AnnIVFFlatTest/AnnIVFFlatTestF_half.AnnIVFFlat/21, where GetParam() = { 1000, 10000, 2051, 16, 40, 1024, distance::InnerProduct, 1,0,0}
 (1920 ms)

@tfeher