[cairo] Documenting how to get test targets to pass that can

[darxus at chaosreigns.com]

I'd like to add some stuff to the test/README to more accurately represent
the current state of tests, and provide instructions to get as many
targets to pass as possible.

How about a section like this, maybe before "Tailoring tests running"
(right after the initial command line examples)?

I'm happy to provide a patch, but I have a feeling some discussion should
happen.


Avoiding tests known to fail
----------------------------

As of August 2016, there are a number of tests known to fail even with
updated references, by generating diffenet output each run, or crashing.
The following will get the maximum number of targets to pass, by using
freshly generated reference images (which should not be necessary).  This
may be useful for generating new references, making changes to cairo, then
running the tests against your references, before submitting your changes.


Xvfb -screen 0 1680x1024x24 -ac -nolisten tcp :2 &

./autogen.sh --enable-xcb
make
# To update reference images, will error:
DISPLAY=:2 CAIRO_TEST_TARGET=xcb,image,image16 make test 
# The new reference images:
cp -a test/output/ $HOME/cairo-ref.xcb-image.xvfb

./autogen.sh --enable-xcb
make
DISPLAY=:2 CAIRO_REF_DIR=$HOME/cairo-ref.xcb-image.xvfb CAIRO_TEST_TARGET=xcb,image,image16 make test