#gpowerd Fix deletion logic in persistent storage.

This CL:
1. Fixes deletion logic in `keepMostRecentProtos`
2. Fix `deleteAllFilesExceptLargest` to correctly delete files
3. Make sure that the tests use a different test directory each time so things are hermetic

PiperOrigin-RevId: 831527582
Change-Id: I2c002350190306ac42b242674901ad214d8653be
3 files changed