parallel.c is not marked as test covered

It seem that it's not covered by the original 924bcf4f commit but I don't know if it's on purpose. This feature being really new that would be understandable.

If it's not, my first idea would be to fix this by adding a simple sql test in /src/test/regress, in the "sql" and "expected" subdirectories that explain (cost off) some queries. 

I'm also guessing here that we can force a query to have a parallel plan with some cost magic to avoid dealing with sufficiently large datasets.