added tests for .pytest_cache README

Helper class to check if readme exists in .pytest_cache directory
 Tests to check for readme when tests pass and when they fail

src/_pytest/cacheprovider.py

@@ -18,7 +18,6 @@ from os.path import sep as _sep, altsep as _altsep
 
 
 class Cache(object):
-
     def __init__(self, config):
         self.config = config
         self._cachedir = Cache.cache_dir_from_config(config)
@@ -106,7 +105,7 @@ class Cache(object):
                 self._ensure_readme()
 
     def _ensure_readme(self):
-        content_readme = '''# pytest cache directory #
+        content_readme = """# pytest cache directory #
 
 This directory contains data from the pytest's cache plugin, \
 which provides the `--lf` and `--ff` options, as well as the `cache` fixture. 
@@ -114,7 +113,7 @@ which provides the `--lf` and `--ff` options, as well as the `cache` fixture.
 **Do not** commit this to version control.
 
 See [the docs](https://docs.pytest.org/en/latest/cache.html) for more information.
-        '''
+        """
         if self._cachedir.check(dir=True):
             readme_path = self._cachedir.join("README.md")
             if not readme_path.check(file=True):
@@ -215,9 +214,7 @@ class NFPlugin(object):
 
             items[:] = self._get_increasing_order(
                 six.itervalues(new_items)
-            ) + self._get_increasing_order(
-                six.itervalues(other_items)
-            )
+            ) + self._get_increasing_order(six.itervalues(other_items))
         self.cached_nodeids = [x.nodeid for x in items if isinstance(x, pytest.Item)]
 
     def _get_increasing_order(self, items):

testing/test_cacheREADME.py

@@ -0,0 +1,42 @@
+from __future__ import absolute_import, division, print_function
+import sys
+import py
+import _pytest
+import pytest
+import os
+import shutil
+
+pytest_plugins = ("pytester",)
+
+
+class Helper(object):
+    def check_readme(self, testdir):
+        config = testdir.parseconfigure()
+        readme = config.cache._cachedir.join("README.md")
+        return readme.isfile()
+
+
+class TestReadme(Helper):
+    def test_readme_passed(self, testdir):
+        testdir.tmpdir.join("test_a.py").write(
+            _pytest._code.Source(
+                """
+            def test_always_passes():
+                assert 1
+        """
+            )
+        )
+        result = testdir.runpytest()
+        assert self.check_readme(testdir) == True
+
+    def test_readme_failed(self, testdir):
+        testdir.tmpdir.join("test_a.py").write(
+            _pytest._code.Source(
+                """
+            def test_always_passes():
+                assert 0
+        """
+            )
+        )
+        result = testdir.runpytest()
+        assert self.check_readme(testdir) == True