Fix tests that assume the backup prefix is a comma.
Steven Winikoff reported to nmh-workers that some tests failed in 1.7
configured with --with-hash-backup because they assume the backup prefix
is comma rather than hash. Alter those tests to use `mhparam sbackup',
as other tests already do. PATH is already set to use the mhparam being
tested.
test-mhfixmsg used find(1) to check for leftover files that globbed
`mhfix*' or `,mhfix*'. Changed to just `*mhfix*' as I think any file
containing that would be unwanted, and it will also spot mhfixmsg
hard-coding the comma prefix.