regtest: Use number of tests run to show the progress

Instead of using the number of document tested. We don't really need to
set a number to a particular document, we only want to know the progress
of the whole process. This ensures that the progress is shown in the
correct order. It also simplifies the code a bit.
Also improved the output formatting to make it easier to read.
regtest: Do not consider docs with no refs as skipped

Handle them differently as new docs, and show them in the summary
suggesting to use create-refs command to include them in the test
suite.
regtest: Do not store the current line in Printer but only its length

We are not using the line text anymore, but only the length.
regtest: Remove unused method _check_exit_status2

It was used when the backends ran in parallel odd and even pages, but
it's no longer used since threads support was added.
regtest: Read test results in chunks to get the md5 digest

Some backends can generate huge results, like huge postscript files that
we don't want to load in memory to get the md5. So, instead of creating
thr md5 object with the entire file, we feed it with chunks of data
using the update method. This improves a lot the memory consumption and
performance as well.
regtest: Do not buffer stderr output

Some buggy documents can produce a huge stderr output because of parsing
errors or whatever. We could give a file directly to Popen to write the
stderr file, but we only want to create the file when there's output,
because it's what we use to know whether the command produced output or
not. So, instead of buffering the whole output and then write it to the
file, now we read from the pipe while the command is running, writing
the output in chunks to the file. This improves a lot the memory
consumption when running some tests.
Limit use of ZapfDingbats character names

Some PDFs use names from ZapfDingbats (a1-a206) without intending for
them to be used for text extraction.  Only use these character names
to locate glyphs or for text extraction with ZapfDingbats fonts.

Bug #60243