Currently, if you try to create in parallel multiple PricebookEntry objects against the StandardPricebook, you will encounter:
System.DmlException: Insert failed. First exception on row 0; first error: UNABLE_TO_LOCK_ROW, unable to obtain exclusive access to this record: 
This is a pretty common operation for @testSetup methods: create a Product2 and create a PricebookEntry with Test.getStandardPricebookId().
Is there really any need to lock the Pricebook when adding a PricebookEntry?