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

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 be7045cdc70d8c026d981997852cc706dab3d5b8
  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.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>