tests/kernel/device: extends test for code coverage

device_get_binding() compares pointers first before doing strcmp().
However, enabling coverage forces -O0 to disable any compiler
optimizations. There would be multiple copies of the same string,
and the code pathing doing pointer comparsion would not be tested
at all. So add this flag to merge string constants such that
the pointer comparison would be exercised.

This also adds a bad driver which fails initialization. This is
to make sure that execution path is covered.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>