bundlespec: fix the "streamv2" and "streamv3-exp" variant
In c4aab3661f25, we broken this feature by adding unicode instead of bytes to
the dictionary.
On the other hand, this feature was never tested, so augment the tests to tests
this.
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)