tox: rm all pyc before doing unit test

Delete python bytecode before every test run.

Because python creates pyc files during tox runs, certain
changes in the tree, like deletes of files, or switching
branches, can create spurious errors.

Closes-Bug: #1368661
Change-Id: I7e563875848acc7ec767f97b31fc8a95b23ee880

tox.ini

@@ -6,11 +6,14 @@ skipsdist = True
 [testenv]
 usedevelop = True
 install_command = pip install -U {opts} {packages}
+whitelist_externals = find
 setenv =
    VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = python setup.py testr --slowest --testr-args='{posargs}'
+commands =
+  find . -type f -name "*.pyc" -delete
+  python setup.py testr --slowest --testr-args='{posargs}'
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:functional]