The helper makes sstable, writes mutations into it and loads one. Internally it uses the make_memtable() helper that prepares a memtable out of a vector of mutations. There are many test cases that don't use these facilities generating some code duplication.
The make_sstable() wrapper around make_sstable_easy() is removed along the way.
Closesscylladb/scylladb#15930
* github.com:scylladb/scylladb:
tests: Use make_sstable_easy() where appropriate
sstable_conforms_to_mutation_source_test: Open-code the make_sstable() helper
sstable_mutation_test: Use make_sstable_easy() instead of make_sstable()
tests: Make use of make_memtable() helper
tests: Drop as_mutation_source helper
test/sstable_utils: Hide assertion-related manipulations into branch
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