Clean up testDir in swirlds-benchmarks

[thenswan]

BaseBench maintains a per-invocation testDir with setTestDir() / getTestDir() / preserveTestDir(),
but only 3 out of 7 benchmark classes actually use it meaningfully:

• DataFileCollectionBench.compaction() — passes getTestDir() to DataFileCollection
• HalfDiskMapBench.merge() — passes getTestDir() to HalfDiskHashMap
• KeyValueStoreBench.merge() — passes getTestDir() to MemoryIndexDiskKeyValueStore

The remaining benchmarks (VirtualMapBench, CryptoBench, ReconnectBench) call setTestDir(...) but
never call getTestDir().