iotests: Replace time.clock() with Timeout

iotests: Replace time.clock() with Timeout

time.clock() is deprecated since Python 3.3. Current Python versions
warn that the function will be removed in Python 3.8, and those warnings
make the test case 118 fail.

Replace it with the Timeout mechanism that is compatible with both
Python 2 and 3, and makes the code even a little nicer.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>