Implemented TestManager.compare_pdfs().

Description: the method functions analogously to TestManager.compare_lys(). The
method implements a naive comparison of a normalized path_1 against a
normalized path_2. PDF normalization amounts to getting rid of lines that
contain any of a handful of strings known to signal document-specific content
(like create date and modification date).

The proscribed content was obtained just by passing a couple of test PDFs to
"diff -a", where the "-a" flag causes diff to treat binary files (like PDFs)
like text files that can be compared line by line.

The method exhibits a simple implementation.

The method comes from frustration attempting to build, install, configure and
test GUI-oriented public solutions.