This series cleans up unit test in preparation for PR #12994.
Helpers are added (or reused) to not rely on specific sstable generation numbers where possible (other than loading reference sstables that are committed to the repo with given generation numbers), and to generate the sstables for tests easily, taking advantage of generation management in `sstable_test_env`, `table_for_tests`, or `replica::table` itself.
Closes#13242
* github.com:scylladb/scylladb:
test: add verify_mutation helpers.
test: add make_sstable_containing memtable
test: table_for_tests: add make_sstable function
test: sstable_test_env: add make_sst_factory methods
test: sstable_compaction_test: do not rely on specific generations
tests: use make_sstable defaults as much as possible
test: sstable_test_env: add make_table_for_tests
test: sstable_datafile_test: do not rely on sepecific sstable generations
test: sstable_test_env: add reusable_sst(shared_sstable)
sstable: expose get_storage function
test: mutation_reader_test: create_sstable: do not rely on specific generations
test: mutation_reader_test: do_test_clustering_order_merger_sstable_set: rely on test_envsstable generation
test: mutation_reader_test: combined_mutation_reader_test: define a local sst_factory function
test: mutation_reader_test: do not use tmpdir
test: use big format by default
test: sstable_compaction_test: use highest sstable version by default
test: test_env: make_db_config: set cfg host_id
test: sstable_datafile_test: fixup indentation
test: sstable_datafile_test: various tests: do_with_async
test: sstable_3_x_test: validate_read, sstable_assertions: get shared_sstable
test: sstable_3_x_test: compare_sstables: get shared_sstable
test: sstable_3_x_test: write_sstables: return shared_sstable
test: sstable_3_x_test: write, compare, validate_sstables: use env.tempdir
test: sstable_3_x_test: compacted_sstable_reader: do not reopen compacted_sst
test: lib: test_services: delete now unused stop_and_keep_alive
test: sstable_compaction_test: use deferred_stop to stop table_for_tests
test: sstable_compaction_test: compound_sstable_set_incremental_selector_test: do_with_async
test: sstable_compaction_test: sstable_needs_cleanup_test: do_with_async
test: sstable_compaction_test: leveled_05: fixup indentation
test: sstable_compaction_test: leveled_05: do_with_async
test: sstable_compaction_test: compact_02: do_with_async
test: sstable_compaction_test: compact_sstables: simplify variable allocation
test: sstable_compaction_test: compact_sstables: reindent
test: sstable_compaction_test: compact_sstables: use thread
test: sstable_compaction_test: sstable_rewrite: simplify variable allocation
test: sstable_compaction_test: sstable_rewrite: fixup indentation
test: sstable_compaction_test: sstable_rewrite: do_with_async
test: sstable_compaction_test: compact: fixup indentation
test: sstable_compaction_test: compact: complete conversion to async thread
test: sstable_compaction_test: compaction_manager_basic_test: rename generations to idx
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