Move Callback tests to platform, USB tests to drivers

[LDong-Arm]

Summary of changes

When refactoring platform and drivers directories, we missed a couple of Greentea test groups. The PR moves the following:

• TESTS/mbed_functional/ -> platform/tests/TESTS/mbed_functional/: tests platform/Callback.h
• TESTS/usb_device/ -> drivers/tests/TESTS/usb_device/: tests drivers/USB*.h

Impact of changes

None.

Migration actions required

None.

Documentation

None.

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@ARMmbed/mbed-os-core

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[ciarmcom]

@LDong-Arm, thank you for your changes.
@ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[rajkan01]

LGTM

[0xc0170]

CI started

[mbed-ci]

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jobs	Status
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[jeromecoutant]

Hi
Did you verify USB tests...?

[1598357855.58][HTST][ERR] host test not detected: pyusb_basic
[1598357867.94][HTST][ERR] host test not detected: usb_device_serial
[1598357879.91][HTST][ERR] host test not detected: usb_device_hid
[1598357892.70][HTST][ERR] host test not detected: pyusb_msd

[jeromecoutant]

Did you verify USB tests...?

See #13501