tests/periph/selftest_shield: release UART after test

Calling `uart_poweroff()` when done with the UART test allows sharing
the underlying hardware e.g. to provide other peripheral interfaces.
One example of this would be the SERCOM3 on the Adafruit Metro M4
Express that is used to provide UART on D1/D0 and SPI on D11/D12/D13.

tests/periph/selftest_shield/main.c

@@ -790,6 +790,9 @@ static bool periph_uart_rxtx_test(uint32_t symbolrate, uint32_t timer_freq)
     failed |= TEST(memcmp(testdata, serial_buf.data, sizeof(serial_buf.data)) == 0);
     failed |= TEST(serial_buf.pos == sizeof(testdata));
 
+    /* disable UART again, in case it is a shared peripheral */
+    uart_poweroff(UART_TEST_DEV);
+
     return failed;
 }