merging and refining examples, also refining skipping documentation.

doc/example/assertion/test_failures.py

@@ -0,0 +1,14 @@
+
+import py
+failure_demo = py.path.local(__file__).dirpath('failure_demo.py')
+pytest_plugins = 'pytester',
+
+def test_failure_demo_fails_properly(testdir):
+    target = testdir.tmpdir.join(failure_demo.basename)
+    failure_demo.copy(target)
+    failure_demo.copy(testdir.tmpdir.join(failure_demo.basename))
+    result = testdir.runpytest(target)
+    result.stdout.fnmatch_lines([
+        "*35 failed*"
+    ])
+    assert result.ret != 0