Test fixture datapath uses relative instead of absoulte file paths
The following code in pandas/blob/master/pandas/conftest.py uses a relative vs absolute path.  

```python
@pytest.fixture
def datapath(request):
    """Get the path to a data file.
    Parameters
    ----------
    path : str
        Path to the file, relative to ``pandas/tests/``
    Returns
    -------
    path : path including ``pandas/tests``.
    Raises
    ------
    ValueError
        If the path doesn't exist and the --strict-data-files option is set.
    """
    def deco(*args):
        path = os.path.join('pandas', 'tests', *args)
        if not os.path.exists(path):
            if request.config.getoption("--strict-data-files"):
                msg = "Could not find file {} and --strict-data-files is set."
                raise ValueError(msg.format(path))
            else:
                msg = "Could not find {}."
                pytest.skip(msg.format(path))
        return path
    return deco
```

This means that the test runner must be run from the root of the project directory which could be inconvenient when running tests in a specific directory.  

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: 36422a88474396148bd7d5d38aa238ea844d9555
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-23-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.0.dev0+170.g36422a884
pytest: 3.6.2
pip: 10.0.1
setuptools: 39.2.0
Cython: 0.28.3
numpy: 1.14.5
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: 1.7.5
patsy: None
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
</details>