Stubs previously in UNITTESTS/target_h/ have the same names as
regular Mbed OS headers, intending to override the latter directly.
We move hal target_h stubs into hal/tests/UNITTESTS/doubles/.

Note: In Mbed OS, the standard include format requires each header to
be prefixed with its module name, for example "hal/gpio_api.h". This
requires headers to be organized in a module directory. But unit tests
stubs for hal correspond to what a real Mbed target would have
implemented (in a non-test scenario), and targets do not currently put
headers in hal/, so we similary put stub headers directly in
hal/tests/UNITTESTS/doubles/ instead of add a hal/ subdirectory there.