test: fix instability in test_ext_viewcode

Use `freshenv` argument and remove `outdir` for one tests
as the previous one can leave an unexpected leftover.

Related: sphinx-doc#11285.

tests/test_ext_viewcode.py

@@ -1,12 +1,13 @@
 """Test sphinx.ext.viewcode extension."""
 
 import re
+import shutil
 
 import pygments
 import pytest
 
 
-@pytest.mark.sphinx(testroot='ext-viewcode')
+@pytest.mark.sphinx(testroot='ext-viewcode', freshenv=True)
 def test_viewcode(app, status, warning):
  app.builder.build_all()
 
@@ -52,6 +53,7 @@ def test_viewcode(app, status, warning):
 
 @pytest.mark.sphinx('epub', testroot='ext-viewcode')
 def test_viewcode_epub_default(app, status, warning):
+ shutil.rmtree(app.outdir)
  app.builder.build_all()
 
  assert not (app.outdir / '_modules/spam/mod1.xhtml').exists()
@@ -83,7 +85,7 @@ def test_linkcode(app, status, warning):
  assert 'http://foobar/cpp/' in stuff
 
 
-@pytest.mark.sphinx(testroot='ext-viewcode-find')
+@pytest.mark.sphinx(testroot='ext-viewcode-find', freshenv=True)
 def test_local_source_files(app, status, warning):
  def find_source(app, modname):
  if modname == 'not_a_package':