Subdevice FOTA Unit tests

[prakharbhatt007]

Mbed Edge pull request

Unit tests for subdevice FOTA feature. Please fill this pull request template to describe your pull request. If it introduces API breaks please describe it in detail.

Description

I have added the unit tests for the subdevice fota feature.

Test instructions

Submitter: Please add instructions to test your PR here.

If your pull request is a fix for a defect please write instructions to reproduce the defect.

Check list

API change(s)

• [ ] Not applicable.
• [ ] API is backwards compatible.
• [ ] API documentation is updated.

Example applications updated

• [ ] Not applicable.