test: Set nonzero exitcode when tests fail

[414owen]

IMPORTANT: Branch is based on os/fix-read-after-show-test, so please merge https://github.com/spl/dlist/pull/117 before this.

• Create a single list of quickcheck properties
• Test the result of the quickcehck run
• Exit with appropriate status code

Previously, cabal test would print PASS and exit with exitcode 0, even when a test was failing.