Fix deficiency in the race test

The test for construction of local static objects doesn't check
to see if the object returned has been fully initialized. Because
of this, an error with ARMCC was not detected.

This patch adds an assert to the race_test to ensure that the
object has been properly initialized.

Author: c1728p9

TESTS/mbed_drivers/race_test/main.cpp

@@ -58,11 +58,13 @@ static SingletonPtr<TestClass> test_class;
 static void main_func_race()
 {
     get_test_class();
+    TEST_ASSERT_EQUAL_UINT32(1, instance_count);
 }
 
 static void main_class_race()
 {
     test_class->do_something();
+    TEST_ASSERT_EQUAL_UINT32(1, instance_count);
 }
 
 void test_case_func_race()