sync::mpsc::select::test::stress is flaky on Windows

[klutzy]

Using Rust rev dcc6ce2, stage1 stdtest.

$ ./stdtest-x86_64-pc-windows-gnu.exe sync::mpsc::select::test::

thread '<unnamed>' panicked at 'assertion failed: i % 2 == 1 && i == i2.unwrap()',
C:\home\stone\rust\src\libstd\sync\mpsc\select.rs:493

test sync::mpsc::select::test::stress ... FAILED

cc #22628, #22710

[klutzy]

Note that there are other channel() tests which are flaky as well.

thread '<unnamed>' panicked at 'assertion failed: `(left == right) && (right == left)`
(left: `2`, right: `4`)', C:\home\stone\rust\src\libstd\sync\mpsc\mod.rs:1494

test sync::mpsc::test::test_recv_iter_break ... FAILED

My hypothesis is channel() is flaky therefore Receiver.recv() may hang even if Sender.send() called.

[klutzy]

Not producible anymore. (see #22710)