test: use full date & time when checking for coredumps

Otherwise we might hit a window where the coredump happens before
midnight, but we check for it after midnight, which yields no results.

E.g.:

```
$ coredumpctl --no-legend --no-pager --file system.journal
Wed 2022-01-05 01:00:06 CET 359 0 0 SIGABRT journal /usr/bin/udevadm n/a
$ coredumpctl --since 23:59:55 --no-legend --no-pager --file system.journal
No coredumps found.
$ coredumpctl --since "2022-01-04 23:59:59" --no-legend --no-pager --file system.journal
Wed 2022-01-05 01:00:06 CET 359 0 0 SIGABRT journal /usr/bin/udevadm n/a
```