Added test coverage result generation

Added test coverage result generation

- added test cases for error and dictionary objects
- utilized dictionary functions for get/set key-value pairs
- clean up generated coverage files on make clean
- enable tests by default if coverage is enabled
- exclude external and generated dbus files from code coverage calculation

Change-Id: Iabcd6f179298301dccc830e6f36ea2aa5e416a8f
Signed-off-by: Imran Zaman <imran.zaman@intel.com>