Move away from getLog in timestamped_reads_wait_for_prepare_oplog_visibility.js

We need to move away from getLog as log events might be truncated and that will make our test flaky.

We need to add another way to wait for the 'prepareTransaction' to complete.

https://github.com/mongodb/mongo/blob/master/jstests/core/txns/timestamped_reads_wait_for_prepare_oplog_visibility.js#L61