tests: uart: uart_async_api: Re-enable tests on xmc45_relax_kit

drivers.uart.async_api.rtt test started failing on xmc45_relax_kit
after f423ec25 which set
CONFIG_SERIAL_SUPPORT_ASYNC=y if uart rtt is enabled. This conflicts
with the xmc4xxx uart driver which sets the same symbol conditionally
if the dma is enabled in the device tree. The conflict created some
warning during compilation causing the CI to fail.

All the uart_async tests were disabled in
71d03947. This commit re-enables the
test on xmc45_relax_kit and only excludes the rtt test that's failing.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>