net/http: run TestRequestWriteTransport with fake time to avoid flakes

This test verifies whether or not we use the chunked encoding when
sending a request with a body like io.NopCloser(strings.NewReader("")).
This depends on whether the transport can read a single byte from the
request body within 200ms, which is flaky on very slow builders.

Use fake time to avoid flakes.

Fixes #52575

Change-Id: Ie11a58ac6bc18d43af1423827887e804242dee30
Reviewed-on: https://go-review.googlesource.com/c/go/+/710737
Auto-Submit: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/net/http/requestwrite_test.go

@@ -15,6 +15,7 @@ import (
 	"strings"
 	"testing"
 	"testing/iotest"
+	"testing/synctest"
 	"time"
 )
 
@@ -667,6 +668,13 @@ func TestRequestWrite(t *testing.T) {
 func TestRequestWriteTransport(t *testing.T) {
 	t.Parallel()
 
+	// Run this test in a synctest bubble, since it relies on the transport
+	// successfully probing the request body within 200ms
+	// (see transferWriter.probeRequestBody).
+	// This occasionally flakes on slow builders (#52575) if we don't use a fake clock.
+	synctest.Test(t, testRequestWriteTransport)
+}
+func testRequestWriteTransport(t *testing.T) {
 	matchSubstr := func(substr string) func(string) error {
 		return func(written string) error {
 			if !strings.Contains(written, substr) {