Add a test utility function, copy_reference_file simply coping
a sample file to the current directory of a running test.
A uudecode filter test will use it for its new test.

TODO: make a symbolic link file insted of copying a sample file.
Historically, libarchive cannot handle uuencoded files when
libarchive's test suit was made. Now libarchive can handle it
directly by uudecode filter, I think, we can reduce copying
sample files to the directory test program expects they are
on an isolated directory for their test.
Fix a bug which uudecode filter wrongly decoded the uuencoded data
which size was more than 64K bytes.