If a .txt file is found next to a given PDF in the "valid" set, take it
as expected text output. Run text extraction on the PDF, saving the
result with extension .txt.out and diff it to the .txt.

Includes a test case for hello.pdf which fails because that file uses a
plain reference for /Contents and we only handle the case where
/Contents is an array.

Includes a second test case hello1.pdf which uses a one-element array
for /Contents but also fails because the code spews diagnostic output to
stdout.