tests/periph/selftest_shield: timer allocation conflict

- Detect when the same timer is used by `ztimer` (pulled in as
  dependency for a peripheral driver, e.g. `periph_adc` on STM32F3) and
  the test application
- Try to provide a better default (e.g. `TIMER_DEV(1)` when
  `ztimer_periph_timer` is in use, `TIMER_DEV(0)` otherwise)

tests/periph/selftest_shield/main.c

@@ -126,7 +126,17 @@
 # define UART_TEST_DEV UART_DEV(0)
 #endif
 #ifndef TIMER
-# define TIMER TIMER_DEV(0)
+# if IS_USED(MODULE_ZTIMER_PERIPH_TIMER)
+# define TIMER TIMER_DEV(1)
+# else
+# define TIMER TIMER_DEV(0)
+# endif
+#endif
+
+#if IS_USED(MODULE_ZTIMER_PERIPH_TIMER)
+# if CONFIG_ZTIMER_USEC_DEV == TIMER
+# error "Same timer used for ztimer and test"
+# endif
 #endif
 
 /* A higher clock frequency is beneficial in being able to actually measure the