cmake: Generate test_keys.h and test_certs.h in the build tree

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/tests/src/certs.c b/tests/src/certs.c
index 879f088..6a01893 100644
--- a/tests/src/certs.c
+++ b/tests/src/certs.c
@@ -13,7 +13,7 @@
 
 #include "mbedtls/pk.h"
 
-#include "test_certs.h"
+#include "test/test_certs.h"
 
 /*
  *