[SV 63044] load: Update .LOADED if the setup function returns -1

[SV 63044] load: Update .LOADED if the setup function returns -1

* src/load.c (load_file): Update .LOADED if setup returns non-0.
* tests/scripts/features/load: Change the return value based on an
environment variable.  Ensure that returning -1 still adds to
.LOADED.  Also add a test that verifies that make doesn't try to
rebuild the loaded file if -1 is returned.