Enable the `enable_tablets` configuration flag in object store tests to better
align with production environments, where it is enabled by default via the
`scylla.yaml` in Scylla's relocatable tarball. This change will improve test
coverage of tablet-related features.
Previously, `enable_tablets` defaulted to false in tests, creating a mismatch
with typical production deployments.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>