Merge: Less randomness and race conditions in tests

Parallel executions can cause some race collisions on named resources (e.g. DB table names).

To solve this issue, `tests.sh` and `nitunit` initialize  `NIT_TESTING_ID` with a distinct integer identifier that can be used to give unique names to resources.

`rand` is not a recommended way to get a distinct identifier because its randomness is disabled by default.

Pull-Request: #2129
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>