ipn/ipnlocal: add GetFilterForTest (#15025)

Needed to test full packet filter in e2e tests. See tailscale/corp#26596

Updates tailscale/corp#20514

Signed-off-by: James Sanderson <jsanderson@tailscale.com>

ipn/ipnlocal/local.go

@@ -1438,6 +1438,10 @@ func (b *LocalBackend) peerCapsLocked(src netip.Addr) tailcfg.PeerCapMap {
 	return nil
 }
 
+func (b *LocalBackend) GetFilterForTest() *filter.Filter {
+	return b.filterAtomic.Load()
+}
+
 // SetControlClientStatus is the callback invoked by the control client whenever it posts a new status.
 // Among other things, this is where we update the netmap, packet filters, DNS and DERP maps.
 func (b *LocalBackend) SetControlClientStatus(c controlclient.Client, st controlclient.Status) {