- When multiple drivers are present, delayed init did not always update
  the handles in the alldrivers such that the drivers were unloaded at
teardown. To fix this, the loader now creates a unique list of driver
library handles opened during the process that need to be released
during context destruction.
- Unit tests added to ensure this functionality continues to operate
  correctly.

Signed-off-by: Neil R. Spruit <neil.r.spruit@intel.com>