GoogleTest allows you to check whether certain function throws errors.
For example:
ASSERT_ANY_THROW(mbed::HeapBlockDevice one(3050, 100));
or
ASSERT_NO_THROW(bd.init());
As MBED_ERROR is now only function that can throw errors, there is
no need to check the type of thrown object.
Allow more than one callback to be register to NetworkInterfaces.
This introduces new APIs:
void NetworkInterface::add_event_listener(...);
void NetworkInterface::remove_event_listener(...);
Which internally calls interfaces attach() functions.
Most functions are empty or simply return "UNSUPPORTED", but it is still worth covering this functions with unit tests to have better control of unwanted changes.