[devtools] change the way we start/stop postgresql Instead of having the test db handler start a cluster on demand, use the test module's setUp/tearDown callbacks to do it. This allows to have one data directory (and thus cluster) per test module, allowing different test modules to run in parallel, each using its own database cluster whose path is based on the test module.

[devtools] make test db name for postgresql process-specific In case we run two tests concurrently on the same cluster, better not use the same db name for both.

[devtools] extract functions to start/stop a postgresql cluster