The lhash test eats machines when developing

[bob-beck]

The lhash test wants to exercise the magic number of 16 threads, hardcoded.

this is not fine on my machine with 14 CPU's (macbook pro) as the 16 threads contend with each other for 80 seconds when running the tests and result in a total make test runtime of 120 seconds.

skipping the lhash test runs all the tests in 49 seconds.

running it with 4 workers runs them in 61 seconds.

this test should not eat machines when not actually measuring performance time in CI. it should let people develop and know they haven't broken the test, but don't use developer pain as measurement. If you care set up CI to run with the correct number of workers you think you need, and measure the runtime over time to ensure
it does not regress in CI with real measurements.

This test should be fixed to not dominate the test time when developers run tests normally.

[jogme]

merged to master.