Allow unit tests to be compiled with -fno-exception

[ladislas]

This PR makes it possible to compile unit tests using the -fno-exception flag.

For some reason, using mbed_assert in unit tests would throw an exception. This is strange as when using mbed on real hardware, exceptions are disabled, thus it's not clear how and why unit test code would use exceptions.

The FAIL() mechanism provided by Google Tests should be sufficient.

But in order to not break user code relying on mbed_assert throwing an exception, we check if exceptions are enabled or not.

The main advantage of running unit tests with -fno-exception is for branch coverage.

With exceptions enabled:

With -fno-exception:

One concrete example is this: we have a PwmOut wrapper called CorePWM. It's simple enough that branch coverage should 100%, but it's not as gcovr tells us that there is a branch on the PwmOut constructor, because it calls init() which in turn will at some point call MBED_ASSERT. This creates a lot of noise for the developers writing unit tests because we don't know if the branch can be controlled or not.

With exceptions enabled:

With -fno-exception:

Another possibility to simplify is to not use throw at all and just rely on the FAIL() in the if/else. I can update the PR if needed.

Good resources on the topic:

• https://stackoverflow.com/questions/29368482/gcov-reports-branches-for-plain-function-calls
• https://stackoverflow.com/questions/23219614/why-gcc-4-1-gcov-reports-100-branch-coverage-and-newer-4-4-4-6-4-8-report

[ciarmcom]

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

[0xc0170]

Just styling, preprocessor macros should start at the beginning

#ifdef
     code....
#else
    code...
#endif

Pull request has been modified.

[ladislas]

Just styling, preprocessor macros should start at the beginning

done ✅

[ciarmcom]

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

[0xc0170]

CI started

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[mergify]

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[0xc0170]

This PR does not contain release version label after merging.

Fixed