Fix flaky test failure in concurrent streams test (#624)

Using a fixed level of parallelism means the test takes too long
when run on a machine with fewer resources/CPUs. So it would
occasionally encounter a timeout-related issue in CI. Fixed by
tying the level of parallelism to GOMAXPROCS (whose default
value comes from the number of CPIs).

connect_ext_test.go

@@ -26,6 +26,7 @@ import (
 	"math"
 	"math/rand"
 	"net/http"
+	"runtime"
 	"strings"
 	"sync"
 	"testing"
@@ -475,7 +476,7 @@ func TestConcurrentStreams(t *testing.T) {
 	server := memhttptest.NewServer(t, mux)
 	var done, start sync.WaitGroup
 	start.Add(1)
-	for i := 0; i < 100; i++ {
+	for i := 0; i < runtime.GOMAXPROCS(0)*8; i++ {
 		done.Add(1)
 		go func() {
 			defer done.Done()

internal/memhttp/memhttp_test.go

@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"runtime"
 	"sync"
 	"testing"
 	"time"
@@ -30,7 +31,7 @@ import (
 
 func TestServerTransport(t *testing.T) {
 	t.Parallel()
-	const concurrency = 100
+	concurrency := runtime.GOMAXPROCS(0) * 8
 	const greeting = "Hello, world!"
 
 	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {