Speed up replset tenant_migration_* tests

There are multiple tenant_migration_*.js tests under the replica sets suite take 10+ minutes. As these tests run a tenant migration hundreds of times surrounding individual user commands, any savings on how long it takes to run a tenant migration can add up.

One observed deficiency is that each tenant migration takes a minimum of 5 seconds to run. There's a problem where majority read concern tailable awaitdata cursors don't return when their first document is ready, but rather when the maxTimeMs is satisfied (default 5 seconds based on the election timeout). Addressing that unnecessary cost alone saves ~10 minutes on every run of tenant_migration_concurrent_writes_on_donor.js.

This ticket should also audit other codepaths to make sure tests are running as expected.