Unify the example programs' termination

This is done to account for platforms, for which we want custom behavior
upon the program termination, hence we call `mbedtls_exit()` instead of
returning from `main()`.
diff --git a/programs/test/selftest.c b/programs/test/selftest.c
index 727054e..43add34 100644
--- a/programs/test/selftest.c
+++ b/programs/test/selftest.c
@@ -418,6 +418,5 @@
         mbedtls_exit( MBEDTLS_EXIT_FAILURE );
 
     /* return() is here to prevent compiler warnings */
-    return( MBEDTLS_EXIT_SUCCESS );
+    mbedtls_exit( MBEDTLS_EXIT_SUCCESS );
 }
-