LibPDF: Add an encoding test file

For now, this uses UTF-16BE and UTF-8 marked strings in page body text.
These markings should be ignored in body text.

Hand-written, with `set fenc=latin1` and `set binary` in vim, and
xref etc fixed up by running

    mutool clean Tests/LibPDF/encoding.pdf  Tests/LibPDF/encoding.pdf

as usual.

Tests/LibPDF/CMakeLists.txt

@@ -9,6 +9,7 @@ endforeach()
 set(TEST_FILES
     colorspaces.pdf
     complex.pdf
+    encoding.pdf
     encryption_nocopy.pdf
     linearized.pdf
     non-linearized.pdf