systimer tests: explicitly initialise SysTimer with lpticker

Now SysTimer needs to be intialised with ticker explicitly set.

Note that lp ticker is still needed for this test as per official
requirement. Tickless from us ticker is only a temporary
workaround for latency issue on some targets.

TESTS/mbedmicro-rtos-mbed/systimer/main.cpp

@@ -53,7 +53,7 @@ class SysTimerTest: public rtos::internal::SysTimer {
 
 public:
     SysTimerTest() :
-        SysTimer(), _sem(0, 1)
+        SysTimer(get_lp_ticker_data()), _sem(0, 1)
     {
     }