feature/featuretags: add test that all ts_omit_foo tags are declared

Updates #12614

Change-Id: I49351fe0c463af0b8d940e8088d4748906a8aec3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_completion_scripts_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_completion_scripts
+
+package buildfeatures
+
+// HasCompletionScripts is whether the binary was built with support for modular feature "embed CLI shell completion scripts".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_completion_scripts" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasCompletionScripts = true