util/cmpver: add a test for comparing three-digit versions

No code changes needed; this is to rule out cmpver as the source of any
version-comparison issues.

Updates #20238

Change-Id: Ib8765dd042e994549d9e2c03859a5f769a856704
Signed-off-by: Alex Chan <alexc@tailscale.com>

util/cmpver/version_test.go

@@ -91,6 +91,12 @@ func TestCompare(t *testing.T) {
 			v2:   "0.96-105",
 			want: 1,
 		},
+		{
+			name: "tailscale-three-digits",
+			v1:   "1.100.2",
+			v2:   "1.55.0",
+			want: 1,
+		},
 		{
 			// Though ۱ and ۲ both satisfy unicode.IsNumber, our previous use
 			// of strconv.ParseUint with these characters would have lead us to