This applies the net package CL 365334, CL 366176, CL 372215 to the os
package.

CL 365334:

    These tests were checking for fairly narrow timing windows, but were
    running in parallel and heavily dependent on timer and goroutine
    scheduling. This change eliminates unnecessary goroutines, runs the
    tests sequentially (dramatically shortening the timeouts to reduce the
    penalty of doing so), and uses timestamp comparison instead of
    background timers to hopefully gain some robustness from monotonic
    timestamps.

    Many of the other tests from this package would benefit from similar
    simplifications, which we can apply if and when we notice flaky
    failures or want to improve the latency of running the test.

CL 366176:

    It appears that at least the OpenBSD kernel gets sloppier the longer
    the timeout we give it, up to an observed overhead of around 25%.
    Let's give it a little more than that (33%) in the comparison, and
    also increase the growth curve to match the actual observed times
    instead of exponential initial growth.

CL 372215:

    Decrease the slop everywhere else, since NetBSD and OpenBSD seem to be
    the only ones that miss by that much.

For golang#36108
For golang#50189
Fixes golang#50725 (we hope)

Change-Id: I0854d27af67ca9fcf0f9d9e4ff67acff4c2effc8
Reviewed-on: https://go-review.googlesource.com/c/go/+/415234
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>