cluster.dtest_alternator_tests.test_slow_query_logging performs
a bootstrap with 768 token ranges.
It works with `me` sstables, which have 2 open file descriptors
per open sstable, but with `ms` sstables, which have 3 open
file descriptors per open sstable, it fails with EMFILE.
To avoid this problem, let's just decrease the number of vnodes
for in the test suite. It's appropriate anyway, because it avoids some
unneeded work without weakening the tests.
(Note: pylib-based have been setting `num_tokens` to 16 for a long time too).
This breaks `bypass_cache_test`, which is written in a way that expects
a certain number of token ranges. We adjust the relevant parameter
accordingly.
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