Disable setting thread priority in TaskPool unittest

https://d.puremagic.com/issues/show_bug.cgi?id=8960

This addresses problem of regular autotester failures, but does not deal with root of the problem. It seems there is no consensus on it. Since attempts to fix the issue failed, supporting unittest which relies on missed (or broken) feature is useless. This makes setting thread priority in TaskPool ctor is no-op since now. 

See also https://github.com/D-Programming-Language/druntime/pull/550