test: drivers: pwm: extends pwm_loopback test

Adds a new zassert to check whether PWM capture has previously been
enabled and correctly returns -EBUSY on a second call.
While working on #39694 this was found to be a missing test case.
The new test case has been verified against the suggested changes of
that PR.

Signed-off-by: Tilmann Unte <unte@es-augsburg.de>