MBED_CONF_APP_DIO_X not well named for PWM tests

I just noticed that MBED_CONF_APP_DIO_X have been added to be used for PWM tests and others.

MBED_CONF_APP_DIO_2 means D2, it makes no sense to re-redefine MBED_CONF_APP_DIO_2 to something else I think, ... clearly I have mis-understood something.

In case of PWM, what we need is to defined a pair PWM-OUT / PWM-IN
where
PWM-OUT is a valid IO for a pwm (MBED_CONF_APP_PWM_0 used for this purpose)
PWM-IN is the IO that is wired with a resistor to PWM-OUT IO on the CI shield

so rather than MBED_CONF_APP_DIO_X, I would propose to use something like MBED_CONF_APP_PWM_0_INT
MBED_CONF_APP_PWM_1_INT
MBED_CONF_APP_PWM_2_INT
MBED_CONF_APP_PWM_3_INT
meaning input interrupt to check PWM_0

this would avoid dependency between PWM tests and BusInOut / InterruptIn tests

Locate the MBED_CONF_APP_DIO_X and MBED_CONF_APP_PWM_0 definitions and inspect the PWM tests and CI shield wiring that use them. Confirm which names represent PWM outputs and wired interrupt inputs; done means the configuration names clearly express those roles without coupling PWM tests to BusInOut or InterruptIn tests.

Written by the indexing model from the issue text.