use synctest to make receive stream tests fully deterministc (#5291)

internal/synctest/synctest_go124.go

@@ -0,0 +1,14 @@
+//go:build go1.24 && !go1.25
+
+package synctest
+
+import (
+	"testing"
+	"testing/synctest"
+)
+
+func Test(t *testing.T, f func(t *testing.T)) {
+	synctest.Run(func() {
+		f(t)
+	})
+}

internal/synctest/synctest_go125.go

@@ -0,0 +1,12 @@
+//go:build go1.25
+
+package synctest
+
+import (
+	"testing"
+	"testing/synctest"
+)
+
+func Test(t *testing.T, f func(t *testing.T)) {
+	synctest.Test(t, f)
+}