In test_xfail_handling, only remove __pycache__ if it exists

Previously, the test failed when the directory was not present,
which could have been caused for example by invoking the tests
with PYTHONDONTWRITEBYTECODE=1.

Fixes https://github.com/pytest-dev/pytest/issues/5664

changelog/5664.trivial.rst

@@ -0,0 +1,2 @@
+When invoking pytest's own testsuite with ``PYTHONDONTWRITEBYTECODE=1``,
+the ``test_xfail_handling`` test no longer fails.

testing/test_stepwise.py

@@ -207,7 +207,8 @@ def test_xfail_handling(testdir):
 
     # because we are writing to the same file, mtime might not be affected enough to
     # invalidate the cache, making this next run flaky
-    testdir.tmpdir.join("__pycache__").remove()
+    if testdir.tmpdir.join("__pycache__").exists():
+        testdir.tmpdir.join("__pycache__").remove()
     testdir.makepyfile(contents.format(assert_value="0", strict="True"))
     result = testdir.runpytest("--sw", "-v")
     result.stdout.fnmatch_lines(