The entire parallel.c reported test coverage is zero: http://coverage.postgresql.org/src/backend/access/transam/parallel.c.gcov.html
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.
