test_filejournal: adjust corrupt entry tests to force header write

The journal no longer assumes corruption if it finds a valid entry
after an inavlid entry.  Instead, these tests will exercise the
corruption detection via the header committed_up_to member.

Fixes: #4792
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>