app/test: fix log check when default level is high

The log unit test was checking display of low priority messages.
It was not working if RTE_LOG_LEVEL is not RTE_LOG_DEBUG.
It is even easier to see since the default level is INFO (9b9d7ca).

Now the test use ERR and CRIT levels which should be always enabled
while not trigerring syslog output on the console.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>