Add regression tests for docwriter.

Docwriter will now be tested with the 'freetype2' repository. The tox environment regression is used, which runs the script tests/freetype.sh, that clones 'freetype2' and configures it. The make targets refdoc and refdoc-venv are tested, and the docs are also built with builddir != srcdir.

Currently the test is allowed to fail (this means that the Travis build will be successful even if the regression test fails), but this can be changed later, so that it becomes necessary for regression tests to pass for the build to be successful.

Note that the testenv regression is not added to [envlist] in tox.ini because we don't want it to run by default when tox command is invoked locally. This is a long test with many requirements, so we prefer to run it only on Travis.