Add \OC_Appconfig Unittest
Add null and emptystring tests to check NOT NULL
Revert "Add null and emptystring tests to check NOT NULL"

This reverts commit c74f3d0b90227f95c1b6d21bbb7ee28561b67446.
Use setUpBeforeClass() and tearDownAfterClass()
Merge pull request #4292 from owncloud/add_appconfig_unittest

Add \OC_Appconfig Unittest