net: eliminate a deadlock on the failure path in TestNotTemporaryRead

net: eliminate a deadlock on the failure path in TestNotTemporaryRead

This fixes a deadlock-on-failure that I probably introduced in
CL 385314.

This doesn't explain why the failure observed in #52492 occurred, but
it will at least give us more information to diagnose the failure if
it happens again. (The deadlock currently prevents the t.Fatal log
message from being written to the test's output.)

Fixes #52492.

Change-Id: I9e7874985e2820a6a4b703abe4f8e2035d5138c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/401575
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>