- fix a copy-paste error (`TIMER_FREQ_UART_TEST` was used in the SPI
test, but that should be `TIMER_FREQ_SPI_TEST`)
- use 400 kHz as slow SPI frequency, as faster STM32 MCUs just cannot
divide the APB clock down to 100 kHz
- when detailed output is enabled, print the SPI clock in addition to
the SPI mode to ease figuring out what went wrong
- only have one `FAILURE` message for a too fast byte transfer per
check, rather than per transmitted byte, to reduce the noise
- work around a bug of `periph_timer` on STM32 by reducing the clock
speed of the timer for the SPI 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.
