tools: restore descending test timing order with --time - #65603

The --time option displays only the first 20 entries after sorting test durations. The current ascending sort therefore reports the fastest tests.

Before 1fc4255221, CompareTime() sorted durations in descending order.
That commit replaced the comparator with an ascending key-based sort while modernizing the Python code.

Sort durations in descending order to restore the historical behavior and report the tests that contribute most to the suite's execution time.

This also matches the output documented in: #27007

The change was verified with:

$ python3 tools/test.py --mode=release --time -j1 -p dots \
    parallel/test-inspector-not-blocked-on-idle \
    parallel/test-http-methods
   1 (00:01.391) release test-inspector-not-blocked-on-idle
   2 (00:00.106) release test-http-methods

AI assistance was used to investigate the Git history and write this description as im not a native english. I reviewed and verified the change.