[FEA] Reduce C++ test runtime

[bdice]

I opened #602 to reduce CAGRA test runtime. The CI jobs currently spend over an hour for that single test (see PR for details).

We should do the same kinds of work to break tests into multiple executables and reduce the parameter sets for these test suites:

• NEIGHBORS_TEST (1486 seconds)
• NEIGHBORS_DYNAMIC_BATCHING_TEST (950 seconds)
• NEIGHBORS_ANN_IVF_PQ_TEST (835 seconds)
• NEIGHBORS_ANN_VAMANA_TEST (505 seconds)
• NEIGHBORS_ANN_NN_DESCENT_TEST (438 seconds)
• NEIGHBORS_ANN_IVF_FLAT_TEST (380 seconds)
  (source for runtimes)