roachtest: use default server.time_after_store_suspect in perturbation

[kvoli]

Previously, server.time_after_store_suspect was set to 10s, down from the default of 30s in perturbation roachtests. It is uncommon for users to alter this cluster setting, let alone decrease it.

Restore the default setting value, 30s.

Part of: https://github.com/cockroachdb/cockroach/issues/132615 Release note: None