Calling `timer_init()` with unsupported frequencies on some MCUs just
selects the closest possible frequency. But e.g. on SAM0, using an
unsupported frequency will cause `timer_init()` to fail; which probably
is the better option.
However, a failing calling to `timer_init()` results in a test failure.
This is now worked around by using timer_query_freq() to select a
suitable timer frequency that is supported.
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.
