Deflake TestTiming in lib/auth
Deflake TestTiming in lib/auth

Two changes to make this more reliable:
- run password checks in parallel to subject both paths to the same
  contention
- do a percentage-based diff (10%) rather than absolute value diff;
  machine load can drastically affect absolute timing diff

Ran on my laptop with 10 parallel test processes, 10 times in a row. All
passed.

Fixes https://github.com/gravitational/teleport/issues/4018

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