test should throw an error if forgot to clone submodules

[bvibber]

If you forget to clone the submodules, the test vectors don't get cloned and the tests "pass" with 0 tests:

$ npm test

> jsivf@0.0.0 test /Users/brion/src/media/jsvpx
> mocha test/test.js --timeout 20000

  0 passing (3ms)

Should probably throw an error here recommending to run 'git submodule update --init'. :)