[FIX] Test for ir_sequence:

This test was depending on the error string returned by postgresql.
As these strings are localized, the test was failing on systems 
where posgresql locale was not set to 'en_US'.

In this new version, we now check the error code returned by postgresql 
(which is, obviously, platform independent)
[REF] Use assertEqual method instead of assert instruction
[MERGE] [FIX] test ir.sequence: when testing the lock mechanism, verify the error based on an error code instead of the error message which is sensible to the locale of the system