pandas.test() issue when run from a development instance of pandas
as described in https://github.com/pydata/pandas/pull/11913#issuecomment-181632169, 
when using a development pandas instance, if `pandas.test()` is called while the working directory of the python interpreter is within the pandas repository, it fails to detect any tests, at least on windows:

```
[pandas_dev] C:\Users\jahfe_000\Documents\GitHub\pandas>python
Python 2.7.11 |Continuum Analytics, Inc.| (default, Jan 29 2016, 14:26:21) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
Anaconda is brought to you by Continuum Analytics.
Please check out: http://continuum.io/thanks and https://anaconda.org
>>> import pandas
>>> pandas.test()
Running unit tests for pandas
pandas version 0.17.1+311.g70f79ce
numpy version 1.10.4
pandas is installed in pandas
Python version 2.7.11 |Continuum Analytics, Inc.| (default, Jan 29 2016, 14:26:21) [MSC v.1500 64 bit (AMD64)]
nose version 1.3.7

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK
<nose.result.TextTestResult run=0 errors=0 failures=0>
>>>
```

if using `python setup.py develop`, a fix is to browse to a folder outside of the repo:

```
[pandas_dev] C:\Users\jahfe_000\Documents\GitHub\pandas>cd ..

[pandas_dev] C:\Users\jahfe_000\Documents\GitHub>python
Python 2.7.11 |Continuum Analytics, Inc.| (default, Jan 29 2016, 14:26:21) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
Anaconda is brought to you by Continuum Analytics.
Please check out: http://continuum.io/thanks and https://anaconda.org
>>> import pandas
>>> pandas.test()
Running unit tests for pandas
pandas version 0.17.1+311.g70f79ce
numpy version 1.10.4
pandas is installed in c:\users\jahfe_000\documents\github\pandas\pandas
Python version 2.7.11 |Continuum Analytics, Inc.| (default, Jan 29 2016, 14:26:21) [MSC v.1500 64 bit (AMD64)]
nose version 1.3.7
C:\Anaconda\envs\pandas_dev\lib\site-packages\matplotlib\backends\qt_compat.py:84: ImportWarning: Not importing directory 'C:\Anaconda\envs\pandas_dev\sip': missing __init__.py
  import sip
....................................S..............S..............S..............S.................F.........................................................................................................................................................
```

This seems to be due to the fact that `pandas.__file__` returns a relative path from the package root when accessed from a folder located within the package, and a relative path when accessed from any other directory.
