ABC-22: Plugin sandboxing for linux/amd64 (#8068)

* plugin sandboxing

* remove unused type

* better symlink handling, better remounting, better test, whitespace
fixes, and comment on the remounting

* fix test compile error

* big simplification for getting mount flags

* mask statfs flags to the ones we're interested in

plugin/rpcplugin/sandbox/supervisor_test.go

@@ -0,0 +1,18 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package sandbox
+
+import (
+	"testing"
+
+	"github.com/mattermost/mattermost-server/plugin/rpcplugin/rpcplugintest"
+)
+
+func TestSupervisorProvider(t *testing.T) {
+	if err := CheckSupport(); err != nil {
+		t.Skip("sandboxing not supported:", err)
+	}
+
+	rpcplugintest.TestSupervisorProvider(t, SupervisorProvider)
+}