tests: drivers: uart: Use EUSART0 as console on xg24_rb4187c

The UART test for USART needs to move the console to an EUSART instance
in order to free up USART0 for the test. Since EUSART1 is configured for
SPI use by the board DTS, use EUSART0 for console.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>