Pass the HOME environment variable to testenv in tox.ini

In order to avoid the "missing HOME variable" issue when running the
unit tests, this patch is passing the HOME environment variable to
[testenv].

Fixes #20424

tox.ini

@@ -16,6 +16,10 @@ commands =
     py27: python -m compileall -fq -x 'test/samples' lib test contrib
     py3{5,6}: python -m compileall -fq -x 'test/samples|lib/ansible/modules' lib test contrib
     make tests
+passenv =
+    # Pass HOME to the test environment to avoid the missing HOME env
+    # variable error. See issue: #20424
+    HOME
 
 
 [flake8]