test the nrf52840 HAL using defmt-test

[japaric]

this PR adds tests that exercise the GPIO API
3 connections are required:

• P0.03 <-> GND
• P0.04 <-> VDD
• P0.28 <-> P0.29

the first two are used to test the Input API against reference voltages
the short-circuit connection is used to test the Input and Output API using the input API that was
previously tested

I tried testing the Uarte API but ran into #289. It seems to me that we can't test TX/RX loopback or overrun errors with the existing read API

[japaric]

moved these into examples/.cargo/config because rustflags in .cargo/config compose and w/o the move I ran into knurling-rs/probe-run#143

[jonas-schievink]

the first two are used to test the Input API against reference voltages

Would it be possible to use the internal pull-up/down resistors to create reference voltages instead? That way it keeps the external wiring to a minimum.

[japaric]

Would it be possible to use the internal pull-up/down resistors to create reference voltages instead? That way it keeps the external wiring to a minimum.

that's a different test in my opinion (and it's already there in the pulldown_is_low and pullup_is_high).

if we do that and remove vdd_is_high and gnd_is_low then we would have less test coverage because we would not be testing the floating input mode independently. (we have the set_low_is_low and set_high_is_high tests but those are testing GPIO output against a GPIO input API that's known to work)

[jonas-schievink]

bors r+

[bors]

Build succeeded:

• ci