The encryption/decryption is implemented asynchronously, so when the unit tests run, the browser will be trying to perform 6 decryptions and 4 encryptions from the official test vectors and 2 random encryption-decryption cycles, all at once!
Author of the decryption code for Casascius' bounty here. I enjoyed having them all run concurrently on my 8-core machine, others may not like it so much.
It should be possible to chain the asynchronous tests so they run serially, and they probably should be chained into the standard tests so the alert at the end can mean the async tests passed too. (There could also be a flag to force scrypt to run synchronously for the tests, but then I would worry about the web-worker asynchronous version not being tested.)Had to add a 5 second wait between each BIP test though because the VM is not fast enough at releasing memory from the previous test - repeatedly ate up all 8gig on my machine lol.