content: include the staleness of the lock when tryLock() fails

When multiple clients are pulling the same image, we may have this
lock error. Short-lived locks are probably fine, but long-lived locks
may indicate that containerd has some issues.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

content/local/locks_test.go

@@ -0,0 +1,32 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package local
+
+import (
+	"testing"
+
+	"gotest.tools/v3/assert"
+)
+
+func TestTryLock(t *testing.T) {
+	err := tryLock("testref")
+	assert.NilError(t, err)
+	defer unlock("testref")
+
+	err = tryLock("testref")
+	assert.ErrorContains(t, err, "ref testref locked since ")
+}