recover_multiple_prepared_transactions_startup.js test should wait for majority commit point to advance before committing a prepared transaction.

Even though the test was running with a single node, after a restart, the lastCommitted is not set until the _firstOpTimeOfMyTerm is durable ( i.e. flushed) and by default we use the lastDurable to calculate the commit point. Therefore, the test races with the WTJournalFlusher thread flushing the first oplog entry after the restart. Committing a prepared transaction with lastCommitted being null is not allowed and will give error "commitTransaction for a prepared transaction cannot be run before its prepare oplog entry has been majority committed".