Increase timing resolution in CacheHashingSpec

[szeiger]

I get times of 0µs vs ~30µs running this test on an M2 Max. The test fails at millisecond precision because usually both values are 0.

[Friendseeker]

I don't quite understand why / 1000000 is not removed from timeUs, but otherwise LGTM.

 def timeUs[R](block: => R): Long = {
    val t0 = System.nanoTime()
    block // call-by-name
    val t1 = System.nanoTime()
    (t1 - t0) / 1000000
  }

Thanks for the PR!

[szeiger]

Huh, you're right, it still measures milliseconds. So it must have been a different problem. I'm not sure why it failed originall, I can't reproduce it anymore.