coverage increase for worker_spi

Tom pointed out that coverage for worker_spi is 0%.  For a module that
only exists to provide coverage, that's pretty stupid.  This patch
increases coverage to 90.9% line-wise and 100% function-wise, which
seems like a sufficient starting point.

How would people feel about me getting this in master at this point in
the cycle, it being just some test code?  We can easily revert if
it seems too unstable.

-- 
Álvaro Herrera