utest: serial: bypass: add depends on RT_USING_SERIAL_BYPASS

Add dependent of RT_USING_SERIAL_BYPASS in Kconfig to
make sure utest for serial-bypass is only available
when RT_USING_SERIAL_BYPASS is enabled.

Otherwise building of utestcases failed when UTEST_SERIAL_BYPASS
is enabled.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

examples/utest/testcases/drivers/serial_bypass/Kconfig

@@ -3,5 +3,6 @@ menu "Serial-Bypass Testcase"
 config UTEST_SERIAL_BYPASS
     bool "Serial testcase"
     default n
+    depends on RT_USING_SERIAL_BYPASS
 
 endmenu