LibXML+Tests: Consume > in the character data ending ]]> and test it

For example, with this input:
```xml
<C>]]>
```
After seeing `<C>`, the parser will start parsing the content of the
element. The content parser will then parse any character data it sees.

The character parser would see the first two `]]` and consume them.
Then, it would see the `>` and set the state machine to say we have
seen this, but it did _not_ consume it and would instead tell
GenericLexer that it should stop consuming characters. Therefore,
we only consumed 2 characters.

Then, it would see that we are in the state where we've seen the
full `]]>` and try to take off three characters from the end of the
consumed input when we only have 2 characters, causing an assertion
failure as we are asking to take off more characters than there really
is.

Tests/CMakeLists.txt

@@ -23,6 +23,7 @@ add_subdirectory(LibTimeZone)
 add_subdirectory(LibUnicode)
 add_subdirectory(LibWasm)
 add_subdirectory(LibWeb)
+add_subdirectory(LibXML)
 if (${SERENITY_ARCH} STREQUAL "i686")
     add_subdirectory(UserspaceEmulator)
 endif()