USB drivers have no unit tests

[rwalton-arm]

Description of defect

drivers/usb test coverage is 0. We should add some unit tests for these drivers.
We should also add unit tests for hal/usb, as the coverage for that folder is 0.

Target(s) affected by this defect ?

N/A

Toolchain(s) (name and version) displaying this defect ?

N/A

What version of Mbed-os are you using (tag or sha) ?

latest

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

N/A

How is this defect reproduced ?

View test coverage for drivers/usb

[ciarmcom]

Thank you for raising this detailed GitHub issue. I am now notifying our internal issue triagers.
Internal Jira reference: https://jira.arm.com/browse/IOTOSM-3952