wgengine/router: fix tests on systems with older Busybox 'ip' binary

Adjust the expected system output by removing the unsupported mask
component including and after the slash in expected output like:
  fwmask 0xabc/0xdef

This package's tests now pass in an Alpine container when the 'go' and
'iptables' packages are installed (and run as privileged so /dev/net/tun
exists).

Fixes #5928

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Id1a3896282bfa36b64afaec7a47205e63ad88542
(cherry picked from commit b63094431b)

wgengine/router/router_linux_test.go

@@ -11,8 +11,10 @@ import (
 	"net/netip"
 	"os"
 	"reflect"
+	"regexp"
 	"sort"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"testing"
 
@@ -341,7 +343,7 @@ ip route add throw 192.168.0.0/24 table 52` + basic,
 			t.Fatalf("failed to set router config: %v", err)
 		}
 		got := fake.String()
-		want := strings.TrimSpace(states[i].want)
+		want := adjustFwmask(t, strings.TrimSpace(states[i].want))
 		if diff := cmp.Diff(got, want); diff != "" {
 			t.Fatalf("unexpected OS state (-got+want):\n%s", diff)
 		}
@@ -922,3 +924,23 @@ func TestCIDRDiff(t *testing.T) {
 		}
 	}
 }
+
+var (
+	fwmaskSupported     bool
+	fwmaskSupportedOnce sync.Once
+	fwmaskAdjustRe      = regexp.MustCompile(`(?m)(fwmark 0x[0-9a-f]+)/0x[0-9a-f]+`)
+)
+
+// adjustFwmask removes the "/0xmask" string from fwmask stanzas if the
+// installed 'ip' binary does not support that format.
+func adjustFwmask(t *testing.T, s string) string {
+	t.Helper()
+	fwmaskSupportedOnce.Do(func() {
+		fwmaskSupported, _ = ipCmdSupportsFwmask()
+	})
+	if fwmaskSupported {
+		return s
+	}
+
+	return fwmaskAdjustRe.ReplaceAllString(s, "$1")
+}