Yes, PostgreSQL needs more tests, but no it is not really a coverage issue. Most of the serious bugs have been related to concurrency or other things which simple test coverage can find. Finding these bugs is usually not trivial.
This is something foundationDB does pretty well. They built a simulator that tests such things. Doubt you could port to Postgres easily though.
That is exactly the kind of tools PostgreSQL needs more of. There are some tools but more are needed. More plain old test coverage will not help much if at all.