tests: detect if debugger is attached

Copy function from libinput/test/litest.c is_debugger_detached()
and use it in our test-runner. If debugger is attached, turn off
leak checks and timeouts automatically.

Revision of libinput: 028513a0a723e97941c39c4aeb17433198723913

v2. rebased to master

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>