tests: Fix running atk-test
- Actually add it to the meson tests.
- Let run_app find app-test when run from the build root by ninja by making it use the build dir.
- It takes almost three minutes on my laptop, fix this by using sub-second sleeps.
- The float values were not exactly representable, leading to build failures on various archs (i386 notably)
Edited by Samuel Thibault