tools: add a libinput test tool as entry point for our test suites

We already install libinput-test-suite if the meson option install-tests is set, see commit be7045cd test: make the test suite runner available as installed binary

To make other tests easily available and more discoverable, add a new tool "libinput test" with the matching man page. This will also help us to enforce some of the namespacing a bit better.