test-journal-flush: do not croak on corrupted input files

test-journal-flush: do not croak on corrupted input files

We would fail if the input file was corrupted:
build/test-journal-flush ./system@0005b7dac334f805-0021aca076ae5c5e.journal\~
journal_file_copy_entry failed: Bad message
Assertion 'r >= 0' failed at src/libsystemd/sd-journal/test-journal-flush.c:55, function main(). Aborting.
[1]    619472 IOT instruction (core dumped)  build/test-journal-flush ./system@0005b7dac334f805-0021aca076ae5c5e.journal\~

Let's skip some "reasonable" errors.

Fixes #17963.