net: simplify deadline fluctuation tests

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.

Fixes #36108

Change-Id: I17d8af7d2eefb1ec14fe0d9d891142a39599a562
Reviewed-on: https://go-review.googlesource.com/c/go/+/365334


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>