net: make two tests not parallel

Running

stress -p 1 go test -short std

on a heavily loaded machine causes net timeouts
every 15 or 20 runs.
Making these tests not run in parallel helps.
With this change, I haven’t seen a single failure
in over 100 runs.

Fixes #14986

Change-Id: Ibaa14869ce8d95b00266aee94d62d195927ede68
Reviewed-on: https://go-review.googlesource.com/21905
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/timeout_test.go

@@ -112,7 +112,6 @@ var dialTimeoutMaxDurationTests = []struct {
 }
 
 func TestDialTimeoutMaxDuration(t *testing.T) {
-	t.Parallel()
 	if runtime.GOOS == "openbsd" {
 		testenv.SkipFlaky(t, 15157)
 	}
@@ -315,8 +314,6 @@ var readTimeoutTests = []struct {
 }
 
 func TestReadTimeout(t *testing.T) {
-	t.Parallel()
-
 	switch runtime.GOOS {
 	case "plan9":
 		t.Skipf("not supported on %s", runtime.GOOS)