ReFrame crashes if verbosity level is increased

[vkarak]

This is the issue causing #1261. Just try the following:

./bin/reframe -c unittests/resources/checks/hellocheck.py -r -vv

The problem seems to come from here, where we're overwriting the record's check_job_completion_time. It seems that the formatting function is called twice and the second time it will have a value (str) that cannot be passed to the converted in the line above.