tests/data: Fix some XML formatting issues in test cases

This allows these test files to pass xmllint.

tests/data/test335

@@ -1,5 +1,5 @@
-# Mostly a duplicate of test168
 <testcase>
+# Mostly a duplicate of test168
 <info>
 <keywords>
 HTTP

tests/data/test338

@@ -1,5 +1,5 @@
-# See https://github.com/curl/curl/issues/4499
 <testcase>
+# See https://github.com/curl/curl/issues/4499
 <info>
 <keywords>
 HTTP

tests/data/test663

@@ -1,8 +1,8 @@
+<testcase>
 #
 # This test is crafted to reproduce oss-fuzz bug
 # https://crbug.com/oss-fuzz/17954
 #
-<testcase>
 <info>
 <keywords>
 HTTP