[CrazyFun] Fixing the issues when running python test targets on Windows

This patch fixes the issues which are happening **only** on Windows, when executing python test targets.

#### Changes made:

1. Error is thrown when running the test targets consecutively with different python versions(py2 to py3 or viz),
    For example,
    `go.bat //py:firefox_test:run method=ColorTests pyversion=C:\python26\python.exe` -> this goes fine

    `go.bat //py:firefox_test:run method=ColorTests pyversion=C:\python33\python.exe` -> throws [error]    (https://gist.github.com/jayakumarc/5354781)

    Issue is happening when virtualenv tries to overwrite the existing python resources under build/python dir. Doing the clean virtualenv install fixes this issue. That is, emptying the build/python dir before new python resources are getting copied.

2. Glob expression in the file path is not interpreted properly when running the test target with Py3.
    (in other words, pytest is not recognizing glob expression in the file/dir path when running under Py3 on Windows)
    For example,
    `go.bat //py:firefox_test:run method=ColorTests pyversion=C:\python33\python.exe`
    throws error,
    ```
    ERROR: file not found: build/lib/selenium/test/selenium/webdriver/firefox/*_tests.py
    go aborted!
    ```
    Specifying test files' pattern in pytest.ini file fixes this issue.


I've tested these changes in Windows7 & Mac-Lion.