[Things-Manager] Added unit test cases.

[Things-Manager] Added unit test cases.

- gtest cases for Things Manager.

Change-Id: I4e4fc5376f006ede62e25fbdabc2eda337d34aa8
Signed-off-by: Jay Sharma <jay.sharma@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/3389
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>
(cherry picked from commit b2e2bdc2d5f3d03ebd0353c7e6f5d5f7a03a938b)
Reviewed-on: https://gerrit.iotivity.org/gerrit/3557
Reviewed-by: Madan Lanka <lanka.madan@samsung.com>