tests: make fuzz-journal-remote less flaky

tests: make fuzz-journal-remote less flaky

by always calling journal_remote_server_destroy, which resets global
variables like journal_remote_server_global. It should prevent crashes like
```
Assertion 'journal_remote_server_global == NULL' failed at src/journal-remote/journal-remote.c:312, function int journal_remote_server_init(RemoteServer *, const char *, JournalWriteSplitMode, _Bool, _Bool)(). Aborting.
AddressSanitizer:DEADLYSIGNAL
=================================================================
==24769==ERROR: AddressSanitizer: ABRT on unknown address 0x0539000060c1 (pc 0x7f23b4d5818b bp 0x7ffcbc4080c0 sp 0x7ffcbc407e70 T0)
SCARINESS: 10 (signal)
    #0 0x7f23b4d5818b in raise /build/glibc-eX1tMB/glibc-2.31/sysdeps/unix/sysv/linux/raise.c:51:1
    #1 0x7f23b4d37858 in abort /build/glibc-eX1tMB/glibc-2.31/stdlib/abort.c:79:7
    #2 0x7f23b5731809 in log_assert_failed systemd/src/basic/log.c:866:9
```