net/dns: add NetworkManager regression test

Use the exact /etc/resolv.conf file from a user report.
Updates https://github.com/tailscale/tailscale/issues/3531

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

net/dns/manager_linux_test.go

@@ -220,6 +220,17 @@ func TestLinuxDNSMode(t *testing.T) {
 			wantLog: "dns: [rc=nm nm-resolved=yes nm=no ret=systemd-resolved]",
 			want:    "systemd-resolved",
 		},
+		{
+			// regression test for https://github.com/tailscale/tailscale/issues/3531
+			name: "networkmanager_but_systemd-resolved_with_search_domain",
+			env: env(resolvDotConf(
+				"# Generated by NetworkManager",
+				"search lan",
+				"nameserver 127.0.0.53"),
+				resolvedRunning()),
+			wantLog: "dns: [rc=nm nm-resolved=yes nm=no ret=systemd-resolved]",
+			want:    "systemd-resolved",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {