net/http: enable TestRequestDoesntBodyCloseDoesntBlock

Fixes #7121.

CL 31173 inadvertently fixed this issue in the Go1.8 cycle.
This CL enables the test for that issue.

Change-Id: I8e73b41f646fa89c75bff833c1b07e1cdb2b1e62
Reviewed-on: https://go-review.googlesource.com/42149Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/serve_test.go

@@ -3702,8 +3702,8 @@ func testTransportAndServerSharedBodyRace(t *testing.T, h2 bool) {
 
 // Test that a hanging Request.Body.Read from another goroutine can't
 // cause the Handler goroutine's Request.Body.Close to block.
+// See issue 7121.
 func TestRequestBodyCloseDoesntBlock(t *testing.T) {
-	t.Skipf("Skipping known issue; see golang.org/issue/7121")
 	if testing.Short() {
 		t.Skip("skipping in -short mode")
 	}