tests: exclude arduion_mkrzero from uart_async and spi_loopback tests

uart_async test and spi_loopback test require UART and SPI
with an ASYNC option.
The SAM0 UART/SPI driver requires configuring DMA support.
Arduino MKRZero is not DMA configured like other SAM0 boards.
We exclude it from the tests.

ea36f9bd and f4c5bdf0 already exclude other SAM0 boards from these tests.
Arduino MKRZero also followed these PR policies.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>