DOC: Update multiple references to testdir to pytester

In https://docs.pytest.org/en/stable/reference.html#testdir, it is
suggested:

> New code should avoid using testdir in favor of pytester.

Multiple spots in the documents still use testdir and they can be quite
confusing (especially the plugin writing guide).

1 files changed, 7 insertions, 7 deletions

diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index 2669cb195..054f809a8 100644
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -324,20 +324,20 @@ Here is a simple overview, with pytest-specific bits:
 Writing Tests
 ~~~~~~~~~~~~~
 
-Writing tests for plugins or for pytest itself is often done using the `testdir fixture <https://docs.pytest.org/en/stable/reference.html#testdir>`_, as a "black-box" test.
+Writing tests for plugins or for pytest itself is often done using the `pytester fixture <https://docs.pytest.org/en/stable/reference.html#pytester>`_, as a "black-box" test.
 
 For example, to ensure a simple test passes you can write:
 
 .. code-block:: python
 
-    def test_true_assertion(testdir):
-        testdir.makepyfile(
+    def test_true_assertion(pytester):
+        pytester.makepyfile(
             """
             def test_foo():
                 assert True
         """
         )
-        result = testdir.runpytest()
+        result = pytester.runpytest()
         result.assert_outcomes(failed=0, passed=1)
 
 
@@ -346,14 +346,14 @@ Alternatively, it is possible to make checks based on the actual output of the t
 
 .. code-block:: python
 
-    def test_true_assertion(testdir):
-        testdir.makepyfile(
+    def test_true_assertion(pytester):
+        pytester.makepyfile(
             """
             def test_foo():
                 assert False
         """
         )
-        result = testdir.runpytest()
+        result = pytester.runpytest()
         result.stdout.fnmatch_lines(["*assert False*", "*1 failed*"])
 
 When choosing a file where to write a new test, take a look at the existing files and see if there's