Unstable test RenderLock1Test always fails on Windows

RenderLock1Test in tests/system is marked as unstable, so is not run by default (unstable tests are only run when passing `-PUNSTABLE_TEST=true` to gradle). On Mac and Linux, this test is in fact unstable in that it passes some times and fails at other times. However, on Windows this test fails all of the time, so the test should be excluded entirely on Windows.