sys/cbor: set isdst to undefined after strptime

CBOR does not specify the daylight saving time adjustment.
tests: fix cbor unittest

Some member of `struct tm val` were not set, resulting in undefined
behavior.
unittests: Fix -Waddress warnings
unittests: Fix -Wunused-parameter warnings
unittests: Fix coding conventions
Merge pull request #1615 from authmillenon/cbor-unittests-warnings

Fix unittests warnings
Merge pull request #1599 from LudwigOrtmann/issue-1597

sys/cbor,tests: fix cbor daylight saving time