feature/routecheck: add ts_omit_routecheck feature flag (#19638)

RouteCheck, which checks that overlapping routers are reachable, is
enabled by default for both tailscaled and tsnet.

Updates #17366
Updates tailscale/corp#33033

Signed-off-by: Simon Law <sfllaw@tailscale.com>

cmd/tailscaled/deps_test.go

@@ -202,6 +202,19 @@ func TestOmitPortlist(t *testing.T) {
 	}.Check(t)
 }
 
+func TestOmitRouteCheck(t *testing.T) {
+	deptest.DepChecker{
+		GOOS:   "linux",
+		GOARCH: "amd64",
+		Tags:   "ts_omit_routecheck,ts_include_cli",
+		OnDep: func(dep string) {
+			if strings.Contains(dep, "routecheck") {
+				t.Errorf("unexpected dep: %q", dep)
+			}
+		},
+	}.Check(t)
+}
+
 func TestOmitGRO(t *testing.T) {
 	deptest.DepChecker{
 		GOOS:   "linux",