runtime: give the subprocess more time in TestSpuriousWakeupsNeverHangSemasleep

Issue #27250 reproduced readily enough to keep the subprocess hung
indefinitely when it occurred, so the timeout does not need to be
short to maintain test fidelity. On the other hand, on heavily loaded
systems it might take a while for the kernel to actually start the
subprocess, and it might also take a while for control flow to return
to the test after the subprocess exits.

We can reduce noise from this test in two ways:

1. Measure the timeout from closer to when the subprocess actually
starts sleeping, instead of when we started creating the subprocess.

2. Use a longer timeout, since it doesn't actually need to be short.

Fixes #38921
Updates #27250

Change-Id: I01c11ae82d0cdc6e7def2da6544b4d07201b35e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/367849


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>