LibPDF: Improve hex string parsing

A local (non-public) PDF I have lying around contains this in
a page's operator stream:

```
[<00b4003e> 3 <002600480051> 3 <005700550044004f0003> -29
<00330044> 3 <0055> -3 <004e0040> 4 <0003> -29 <004c00560003> -31
<0057004b> 4 <00480003> -37 <0050
>] TJ
```

That is, there's a newline in a hexstring after a character.

This led to `Parser error at offset 5184: Unexpected character`.

The spec says in 3.2.3 String Objects, Hexadecimal Strings:
"""Each pair of hexadecimal digits defines one byte of the string.
White-space characters (such as space, tab, carriage return, line feed,
and form feed) are ignored."""

But we didn't ignore whitespace before or after a character, only
in between the bytes.

The spec also says:
"""If the final digit of a hexadecimal string is missing—that is, if
there is an odd number of digits—the final digit is assumed to be 0."""

In that case, we were skipping the closing `>` twice -- or, more
accurately, we ignored the character after it too. This has been
wrong all the way back in #6974.

Add a test that fails if either of the two changes isn't present.

Userland/Libraries/LibPDF/Parser.cpp

@@ -352,6 +352,7 @@ PDFErrorOr<ByteString> Parser::parse_hex_string()
     StringBuilder builder;
 
     while (true) {
+        m_reader.consume_whitespace();
         if (m_reader.matches('>')) {
             m_reader.consume();
             return builder.to_byte_string();
@@ -364,7 +365,6 @@ PDFErrorOr<ByteString> Parser::parse_hex_string()
                 if (ch == '>') {
                     // The hex string contains an odd number of characters, and the last character
                     // is assumed to be '0'
-                    m_reader.consume();
                     hex_value *= 16;
                     builder.append(static_cast<char>(hex_value));
                     return builder.to_byte_string();