test: wait for coredump to appear before parsing

test: wait for coredump to appear before parsing

A new core was added to the test, but the loop counter was not increased
to wait for it, so the test races against systemd-coredump's processing.

This failed at least once in debci:

8015s [   32.227813] TEST-87-AUX-UTILS-VM.sh[1038]: + coredumpctl info COREDUMP_TIMESTAMP=1679509902000000
8015s [   32.228684] TEST-87-AUX-UTILS-VM.sh[1723]: No coredumps found.

Follow-up for 0c49e0049b7665bb7769a13ef346fef92e1ad4d6

Fixes https://github.com/systemd/systemd/issues/37666