Document how to run the project tests, the unit tests, and both together

Must document:

• That CI runs on Appveyor (and what platforms, with a link to https://ci.appveyor.com/project/jpakkane/meson/history) and Travis (link to https://travis-ci.org/mesonbuild/meson/builds)
• ./run_tests.py runs all tests, including cross-tests on Ubuntu (which we need to extend to other distros)
• ./run_project_tests.py runs project tests, what their purpose is, and the fact that right now you cannot run each individually (we should also fix that)
• ./run_unittests.py runs unit tests, which ones run on which platform (so people know where to put new tests), how to run individual unit tests

More.