Revert "tests/mutation_source_tests: generate_mutation_sets() use larger ttl"
Revert "tests/mutation_source_tests: generate_mutation_sets() use larger ttl"

This reverts commit cea3338e388f4091f7cfdb70427ab7549fab5e36.

The above was a quick fix to allow the tests to pass, there is a proper
fix now.

avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla