When a cram test attempts to read stdin, it will read from dune's stdin. I think it should be closed instead.

A simple way to reproduce this is to run this block in a cram test (e.g. in otherlibs/cram/test/run.t):

$ cat

Then, dune runtest hangs. If some input is fed to dune, such as typing "hello" followed by Ctrl-D, the test suggests a promotion:

 $ cat
+hello

This happens on master as of 4c79d96.

(cc @rgrinberg, I found this while testing the cram stanza but this is not related)

Thanks!