Make sure unit tests can be run in parallel

Julien:
> user rights issue ?
> http://qa.nuxeo.org/hudson/job/Nuxeo%205.2%20-%20Nuxeo%20Core%201.5%20(Freestyle)/978/testReport/
> "org.h2.jdbc.JdbcSQLException: The database is read only"
> The database was apparently stored in /tmp/nxsqltests-h2/, could someone check how it should work ? Eventually same fix as on cache converter could be applied.

Florent:
this is a bit more complex, as this path is inside a fixed xml file in a contribution:
nuxeo-core-storage-sql/src/test/resources/OSGI-INF/test-repo-repository-h2-contrib.xml
and it needs to be known by the java code to do cleanups:
nuxeo-core-storage-sql/src/test/java/org/nuxeo/ecm/core/storage/sql/SQLBackendHelper.java
Indeed if several tests are run at the same time on the same machine it's gonna be a problem.