The test looks at metrics to confirm whether queries
hit the row cache, the index cache or the disk,
depending on various settings.
BIG index readers use a two level, read-through index cache,
where the higher layer stores parsed "index pages"
of Index.db, while the lower layer is a cache of
raw 4kiB file pages of Index.db.
Therefore, if we want to count index cache hits,
the appropriate metric to check in this case is
`scylla_sstables_index_page_hits",
which counts hits in the higher layer.
This is done by the test.
However, BTI index readers don't have an equivalent of the higher cache
layer. Their cache only stores the raw 4 kiB pages, and the hits are
counted in `scylla_sstables_index_page_cache_hits`. (The same metric
is incremented by the lower layer of the BIG index cache).
Before this commit, the test would fail with `ms` sstables,
because their reads don't increment `scylla_sstables_index_page_hits`.
In this commit we adapt the test so that it instead checks
`scylla_sstables_index_page_cache_hits` for `ms` sstables.
Use universalasync library to make test.py async code compatible
with synchronous code of dtest/ccm
Also, copied unmodified error_example_test.py from dtest as an example.
Run the test in `dev` mode only.
