Add missing tests for instance_type_extra_specs_* methods

There was no tests in test_db_api for instance_type_extra_specs_*
methods.

Add tests to ensure that all works.

blueprint db-api-tests

Change-Id: Ie88aa776574c5ddf37a1adcf7cc2c9e70c6eb810
Merge "Add missing tests for instance_type_extra_specs_* methods"