Stabilise unit tests for generateSeed

[slaweet]

Expected behaviour

All unit tests pass each time

Actual behaviour

Sometimes this happes

FAILED TESTS:
  Passphrase
    generateSeed
      ✖ should generate a predictable sequence of bytes for given list of randoms
        Chrome 59.0.3071 (Mac OS X 10.12.6)
      AssertionError: expected [ Array(16) ] to deeply equal undefined
          at Assertion.assertEqual (src/utils/passphrase.test.js:62131:19)
          at Assertion.ctx.(anonymous function) [as equal] (src/utils/passphrase.test.js:65569:25)
          at src/utils/passphrase.test.js:77832:48
          at Array.forEach (<anonymous>)
          at Context.<anonymous> (src/utils/passphrase.test.js:77829:15)

Steps to reproduce

Run unit tests several times