fixes VaultSecrets test - it was failing due to a
regex mismatch (`^job` stopped matching when
copywrite headers got prepended to the jobspec).
but RegisterFromJobspec (which had the bug)
was only used in the one spot, so instead this
refactors the whole test to the v3 format
with testing.T and some additional fun stuff
that we can take advantage of with it.
some improvements:
* use a namespace
* use and extend existing test helpers
* add more test helpers
