Make it easier for cairo-test-suite to find the source dir

The view-test-results.py script can not read the reference images because they are outside the directory served by the http server.

This MR creates a "srcdir" symlink in the build dir to the src test dir and updates cairo-test to look for this symlink. It also changes the "srcdir" env var to CAIRO_SRC_DIR to be consistent with the other test env vars and documents it in the README.