internal/goexperiment: run go generate for synctest

Updates #70954

Change-Id: If5f9c8b8b820b1cc4e41e76b50038c6155b575a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/638355
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/internal/goexperiment/exp_synctest_off.go

@@ -0,0 +1,8 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.synctest
+
+package goexperiment
+
+const Synctest = false
+const SynctestInt = 0

src/internal/goexperiment/exp_synctest_on.go

@@ -0,0 +1,8 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.synctest
+
+package goexperiment
+
+const Synctest = true
+const SynctestInt = 1