make unittests less verbose

make unittests less verbose

Right now, the unittests are verbose and produce a lot of outputs. This
makes it difficult to debug them.

This patch sets different levels of logging: debug, info, warning and
error. By default, debug messages aren't showed anymore. That way, it
will be easier to see when something actually goes wrong in a test.

It's possible to display them again by setting the environment variable
UTC_MIN_LOG to DEBUG

Reviewed by: a1.gomes, hh4813.kim, sns.park

Change-Id: I6f4e8d61ff5ec24eaccec5240d3fa106edd8ff7c
Signed-off-by: Arnaud Renevier <a.renevier@samsung.com>