Increased wait time to allow device to sleep

With small wait time at the start, chances are that device does not
enter sleep and idle time is zero. Increasing wait time to make sure
device goes to sleep.

Author: deepikabhavnani

TESTS/mbed_platform/stats_cpu/main.cpp

@@ -65,7 +65,7 @@ void test_cpu_info(void)
     mbed_stats_cpu_t stats;
     // Additional read to make sure timer is initialized
     mbed_stats_cpu_get(&stats);
-    Thread::wait(1);
+    Thread::wait(3);
     mbed_stats_cpu_get(&stats);
     TEST_ASSERT_NOT_EQUAL(0, stats.uptime);
     TEST_ASSERT_NOT_EQUAL(0, stats.idle_time);