chore: add comment about overlay imports

Signed-off-by: Norman <[email protected]>

gnovm/tests/stdlibs/README.md

@@ -4,3 +4,5 @@ This directory contains test-specific standard libraries. These are only
 available when testing gno code in `_test.gno` and `_filetest.gno` files.
 Re-declarations of functions already existing override the definitions of the
 normal stdlibs directory.
+
+Adding imports that don't exist in the corresponding normal stdlib is undefined behavior