test: Fix function does not return a value warnings

[Patater]

Summary of changes

Many test stub functions are meant to return a value, but weren't. Clang would generate a warning for each instance where we weren't returning anything in a function that was meant to return a value.

        warning: non-void function does not return a value [-Wreturn-type]

For a specific example, my_radio::time_on_air() is supposed to return a uint32_t, but wasn't returning anything. We'll return a zero instead of relying on undefined behavior.

Without this, clang 11.0.1 was generating a virtual function implementation with a ud2 instruction to abort at run-time, causing some execution of some unit tests to abort.

        Running main() from gmock_main.cc
        [==========] Running 10 tests from 1 test suite.
        [----------] Global test environment set-up.
        [----------] 10 tests from Test_LoRaPHYUS915
        [ RUN      ] Test_LoRaPHYUS915.constructor
        [       OK ] Test_LoRaPHYUS915.constructor (0 ms)
        [ RUN      ] Test_LoRaPHYUS915.restore_default_channels
        [       OK ] Test_LoRaPHYUS915.restore_default_channels (0 ms)
        [ RUN      ] Test_LoRaPHYUS915.rx_config
        [       OK ] Test_LoRaPHYUS915.rx_config (0 ms)
        [ RUN      ] Test_LoRaPHYUS915.tx_config
        Process 35669 stopped
        * thread #1, name = 'lorawan-loraphy-', stop reason = signal SIGILL: privileged instruction
            frame #0: 0x0000000000276f73 lorawan-loraphy-us915-unittest`my_radio::time_on_air(this=0x0000000800c2b048, modem=MODEM_LORA, pkt_len='\0') at Test_LoRaPHYUS915.cpp:90:5
           87       };
           88
           89       virtual uint32_t time_on_air(radio_modems_t modem, uint8_t pkt_len)
        -> 90       {
           91       };
           92
           93       virtual bool perform_carrier_sense(radio_modems_t modem,
        (lldb) disassemble --pc
        lorawan-loraphy-us915-unittest`my_radio::time_on_air:
        ->  0x276f73 <+67>: ud2
            0x276f75:       int3
            0x276f76:       int3
            0x276f77:       int3
        (lldb)

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

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[ciarmcom]

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

[LDong-Arm]

LGTM. Please fix indentation in the PR description (e.g. copy the commit message from the editor during amend, instead of the git log).

[Patater]

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