net: make TestDialFailPDLeak shorter

Reduces a number of trials but it still can detect memory leak
when we make blunders in runtime-integarted network poller work,
like just forgetting to call runtime_pollClose in code paths.

Also disables the test on windows/386.

R=alex.brainman, r
CC=golang-dev
https://codereview.appspot.com/13022046