Request Limiter Reload tests (#25126)

This PR introduces a new testonly endpoint for introspecting the
RequestLimiter state. It makes use of the endpoint to verify that changes to
the request_limiter config are honored across reload.

In the future, we may choose to make the sys/internal/request-limiter/status
endpoint available in normal binaries, but this is an expedient way to expose
the status for testing without having to rush the design.

In order to re-use as much of the existing command package utility funcionality
as possible without introducing sprawling code changes, I introduced a new
server_util.go and exported some fields via accessors.

The tests shook out a couple of bugs (including a deadlock and lack of
locking around the core limiterRegistry state).

command/server_util.go

@@ -0,0 +1,48 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: BUSL-1.1
+
+package command
+
+import (
+	"testing"
+
+	"github.com/hashicorp/cli"
+	"github.com/hashicorp/vault/sdk/physical"
+	physInmem "github.com/hashicorp/vault/sdk/physical/inmem"
+)
+
+func TestServerCommand(tb testing.TB) (*cli.MockUi, *ServerCommand) {
+	tb.Helper()
+	return testServerCommand(tb)
+}
+
+func (c *ServerCommand) StartedCh() chan struct{} {
+	return c.startedCh
+}
+
+func (c *ServerCommand) ReloadedCh() chan struct{} {
+	return c.reloadedCh
+}
+
+func testServerCommand(tb testing.TB) (*cli.MockUi, *ServerCommand) {
+	tb.Helper()
+
+	ui := cli.NewMockUi()
+	return ui, &ServerCommand{
+		BaseCommand: &BaseCommand{
+			UI: ui,
+		},
+		ShutdownCh: MakeShutdownCh(),
+		SighupCh:   MakeSighupCh(),
+		SigUSR2Ch:  MakeSigUSR2Ch(),
+		PhysicalBackends: map[string]physical.Factory{
+			"inmem":    physInmem.NewInmem,
+			"inmem_ha": physInmem.NewInmemHA,
+		},
+
+		// These prevent us from random sleep guessing...
+		startedCh:         make(chan struct{}, 5),
+		reloadedCh:        make(chan struct{}, 5),
+		licenseReloadedCh: make(chan error),
+	}
+}