net: TestDialerDualStack is flaky

[tzneal]

It's been failing regularly on Windows/x64 , example at https://build.golang.org/log/6d9c92c981b5fb561f33eda94a418ee2ec05a846

--- FAIL: TestDialerDualStack (4.91s)
    dial_test.go:664: dial tcp 127.0.0.1:4306: i/o timeout

[mikioh]

/CC @pmarks-net

Same as #12052, dialClosedPort on Windows also takes a bit long time?

[gopherbot]

CL https://golang.org/cl/14120 mentions this issue.

[rsc]

Fixed by CL 14120.