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: If31ffc245ade9f62b61f99246e59208208ed4fb1

tox.ini

@@ -7,6 +7,7 @@ envlist = py27,pep8
 usedevelop = True
 
 install_command = pip install -U {opts} {packages}
+whitelist_externals = find
 setenv =
    VIRTUAL_ENV={envdir}
    LANG=en_US.UTF-8
@@ -14,7 +15,9 @@ setenv =
    LC_ALL=en_US.utf-8
 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}'
 
 [testenv:pep8]
 commands = flake8