Throw errors when the test database cannot be accessed/created
EZP-22814: Fix tests database automatic creation
ezpTestDatabaseHelper::create() is designed to create the tests database.
But the way to connect to the db when checking for tests db existence is wrong.
We must not try to select the database name provided in the dsn, because if it does not exist, the eZDB instance won't be valid and the check for existence will fail.
Fix dsn argument documentation example
Merge pull request #954 from guillaumelecerf/EZP-22814

EZP-22814: Fix tests database automatic creation