tests: remove entangling corpus index optimization

tests: remove entangling corpus index optimization

The add_email_corpus test utility includes logic that tries to re-use
an index of the corpus if available.  This was seemingly done as an
optimization, so that every test that uses the corpus didn't have to
create it's own index of the corpus.  However, this has the perverse
side effect of entangling tests together, and breaks parallelization.

Forcing each test to do it's own index does increase the overall time
of the test slightly (~6%), but this will be more than made up for in
the next patch that introduces paraellization.