tests/pyb/uart: Update test to match recent change to UART timeout_char.

tests/pyb/uart.py.exp

@@ -12,8 +12,8 @@ UART XB
 UART YA
 UART YB
 ValueError Z
-UART(1, baudrate=9600, bits=8, parity=None, stop=1, timeout=1000, timeout_char=2, read_buf_len=64)
-UART(1, baudrate=2400, bits=8, parity=None, stop=1, timeout=1000, timeout_char=6, read_buf_len=64)
+UART(1, baudrate=9600, bits=8, parity=None, stop=1, timeout=1000, timeout_char=3, read_buf_len=64)
+UART(1, baudrate=2400, bits=8, parity=None, stop=1, timeout=1000, timeout_char=7, read_buf_len=64)
 0
 3
 4