Additional Tests for Batch Descriptor Inserts

[ifadams]

Is your feature request related to a problem? Please describe.
Our batch descriptor insert feature (#209) needs more integrated testing for automation and regression checks.

Describe the solution you'd like
We should develop additional end-to-end tests exercising different descriptor engines, as well as additional unit tests to improve coverage.

[cwlacewe]

To keep track of what's needed:

• Coverage difference is in src/DescriptorsCommand.cc where 64 lines of code was added but none tested:

• Gap should be resolved by making sure there are C++ tests for AddDescriptor::add_single_descriptor and AddDescriptor::add_descriptor_batch