Return from the test and utility examples via exit()

All the core examples have been modified not to return from main
by the means of the return statement, but rather via exit() function,
which was done to make the examples more bare metal friendly.

This commit, for the sake of consistency, introduces the modifications
to the test and utility examples. These, while less likely to be used
in the low level environments, won't suffer from such a change.

programs/util/pem2der.c

@@ -61,7 +61,7 @@
 int main( void )
 {
     mbedtls_printf("MBEDTLS_BASE64_C and/or MBEDTLS_FS_IO not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -290,6 +290,6 @@ exit:
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_BASE64_C && MBEDTLS_FS_IO */