Add UT to test BG read qps behavior during upgrade for pr11406

[hx235]

Context/Summary:
When db is upgrading to adopt pr11406, it's possible for RocksDB to infer a small tail size to prefetch for pre-upgrade files. Such small tail size would have caused 1 file read per index or filter partition if partitioned index or filer is used. This PR provides a UT to show this would not happen.

Misc: refactor the related UTs a bit to make this new UT more readable.

Test:

• New UT
  If logic of upgrade is wrong e.g,

--- a/table/block_based/partitioned_index_reader.cc
+++ b/table/block_based/partitioned_index_reader.cc
@@ -166,7 +166,8 @@ Status PartitionIndexReader::CacheDependencies(
   uint64_t prefetch_len = last_off - prefetch_off;
   std::unique_ptr<FilePrefetchBuffer> prefetch_buffer;
   if (tail_prefetch_buffer == nullptr || !tail_prefetch_buffer->Enabled() ||
-      tail_prefetch_buffer->GetPrefetchOffset() > prefetch_off) {
+      (false && tail_prefetch_buffer->GetPrefetchOffset() > prefetch_off)) {

, then the UT will fail like below

[ RUN      ] PrefetchTailTest/PrefetchTailTest.UpgradeToTailSizeInManifest/0
file/prefetch_test.cc:461: Failure
Expected: (db_open_file_read.count) < (num_index_partition), actual: 38 vs 33
Received signal 11 (Segmentation fault)

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[hx235]

Resurrect a sync point that no test used

[pdillinger]

Thanks for the test!

[pdillinger]

Ideally this would not be needed, as it overrides some of the logic, but I suppose it is OK. It's probably tricky to reliably force the heuristics to be wrong the way we want.

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@hx235 merged this pull request in 1da9ac2.