vcs-test/vcweb: add a reminder to delete vcs-test when no longer needed

After CL 427914 is merged, the tests for cmd/go will no longer depend
on the server hosted at vcs-test.golang.org. The release branch for Go
1.19 will continue to need that server until Go 1.19 is no longer
supported, but after that point the server can be permanently turned
down and its code deleted.

The test in this CL will remind us to do that deletion by failing on
the -longtest builders when the Go version in the main repo is updated
for the Go 1.22 development cycle. The Go 1.22 cycle will begin after
Go 1.21 is released, and (assuming our current release policy remains
in effect) that coincides with the end of support for Go 1.19.

Updates golang/go#27494.

Change-Id: I1e98b88060e303ae75e2f3fb86d6f34bd7765326
Reviewed-on: https://go-review.googlesource.com/c/build/+/428835
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

vcs-test/vcweb/deathclock_go121_test.go

@@ -0,0 +1,13 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.22
+
+package main_test
+
+func init() {
+	// When development begins for the Go 1.22 cycle, the supported Go
+	// releases will be Go 1.20 and 1.21.
+	go119Unsupported = true
+}

vcs-test/vcweb/deathclock_test.go

@@ -0,0 +1,27 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import "testing"
+
+var go119Unsupported = false
+
+var turndownMsg = `
+Since Go 1.19 is not longer supported, vcs-test.golang.org is no
+longer needed for testing any release branch and should be turned
+down, and x/build/vcs-test/... should be deleted.
+(See https://go.dev/issue/27494.)`
+
+func TestTurnDownVCSTest(t *testing.T) {
+	if !go119Unsupported {
+		return
+	}
+
+	if testing.Short() {
+		t.Log(turndownMsg)
+	} else {
+		t.Fatal(turndownMsg)
+	}
+}