binman: Don't change the descriptor in tests

At present testPackX86RomMeNoDesc removes the contents of the
descriptor.bin file and testPackX86RomMeMissingDesc removes the file
completely.

If a test that relies on this file happens to run after it is removed, it
will not work. Since we have no control over the selecting of tests that
run in parallel and series, we must avoid changing the files.

Update this tests to use separate files instead.

Signed-off-by: Simon Glass <sjg@chromium.org>