Add self-crashing test

src/tests/test_crashing.py

@@ -0,0 +1,20 @@
+import os
+
+import pytest
+
+
+@pytest.mark.skip
+def test_crash():
+    """
+    This is a test that crashes with SIGKILL every (n+1)-th time it runs (n = 0, 1, 2, …).
+    This is useful for debugging our pytest-xdist monkeypatch that we apply in conftest.py
+    to deal with random test crashes on Travis CI using SQLite. Usually, this test is
+    skipped to avoid causing additional crashes in real runs.
+    """
+    if os.path.exists('crashed.tmp'):
+        assert 1
+        os.remove('crashed.tmp')
+    else:
+        with open('crashed.tmp', 'w') as f:
+            f.write('hi')
+        os.kill(os.getpid(), 9)