net: add even more timing slop for TestDialParallel
net: add even more timing slop for TestDialParallel

This test expects dials of a closed port to complete in about the same
amount of time: an initial probe value +/- 20%. Reduce test flakes on
Windows by increasing the slop to +/- 50% of the original value.

Fixes #52173

Change-Id: I813492c36aca2b0264b3b5b8c96e8bf97193af76
Reviewed-on: https://go-review.googlesource.com/c/go/+/408354
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,golang/go,golang/go,nawawi/go,golang/go,golang/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go