swaglog: delete oldest (not newest) logs on rotation after restart

[PeterPhuTran]

Description

SwaglogRotatingFileHandler maintains its file list newest-first — _open() prepends
each new file with insert(0, ...) and doRollover() prunes past backup_count
with pop() from the tail — but get_existing_logfiles() seeded the list
oldest-first. After any process restart at the 2500-file cap, the tail therefore
held the newest pre-existing file, and every rollover deleted the most recently
written logs, marching backwards through them, while months-old files survived.

Field evidence: on a real comma three, every boot destroyed the newest ~100+ swaglog
files from before the reboot — exactly the files needed to debug whatever caused the
reboot. This bug erased weeks of crash forensics during the #309 investigation
(logs from the failing boots kept vanishing days after being written).

Fix: one word — seed the list newest-first (sorted(log_files, reverse=True))
to match the insert(0)/pop() convention.

Verification

New common/tests/test_swaglog.py:

• fails on the unpatched base (restart prune deletes the wrong files), passes with the fix
• covers the restart prune, repeated rollovers, and a second restart over survivors
• verified on-device (comma three, red-on-bug → green-on-fix)

The same bug exists in upstream commaai/openpilot; submitted there separately.

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