tests-mbed_drivers-timerevent: Disable deep-sleep mode in TestTimerEvent class.

Since now us ticker is disabled in deep-sleep mode and tests-mbed_drivers-timerevent verifies TimerEvent class which uses us ticker then deep-sleep mode needs to be disabled during the test execution.

TESTS/mbed_drivers/timerevent/main.cpp

@@ -43,6 +43,8 @@ private:
 public:
     TestTimerEvent() :
             TimerEvent(), sem(0, 1) {
+
+        sleep_manager_lock_deep_sleep();
     }
 
     TestTimerEvent(const ticker_data_t *data) :
@@ -50,6 +52,7 @@ public:
     }
 
     virtual ~TestTimerEvent() {
+        sleep_manager_unlock_deep_sleep();
     }
 
     // Make these methods publicly accessible