The test should execute only with `make test-with-config` and not with
`make test`, as boards without the shield cannot pass the test.
For some reason I accidentally added both variants, which makes no
sense. This drops the `make test` variant.
Finally, the `README.md` is updated to refer to `make test-with-config`
instead of `make test`.
This test application makes use of the RIOT Peripheral Selftest Shield,
which connects e.g. PWM to ADC or SPI MOSI to SPI MISO, UART TXD to RXD,
etc. This provides quick and fully automated self testing capabilities.
Please note that the simplicity and ease of use of the hardware comes
with a prices: There are whole classes of issues that cannot be detected
automatically. This test cannot replace other testing approaches
(such as manual testing or PHiLIP on the HiL), but only complement them.
