test_formats.py not parametrized
#### Code Sample, a copy-pastable example if possible

```python

def test_repr():
    assert (repr(Timedelta(10, unit='d')) ==
            "Timedelta('10 days 00:00:00')")
    assert (repr(Timedelta(10, unit='s')) ==
            "Timedelta('0 days 00:00:10')")
    assert (repr(Timedelta(10, unit='ms')) ==
            "Timedelta('0 days 00:00:00.010000')")
    assert (repr(Timedelta(-10, unit='ms')) ==
            "Timedelta('-1 days +23:59:59.990000')")
```
#### Problem description

should be parameterised.

#### Expected Output

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

<details>

INSTALLED VERSIONS
------------------
commit: 01882ba5b4c21b0caf2e6b9279fb01967aa5d650
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.23.0.dev0+657.g01882ba5b
pytest: 3.3.2
pip: 9.0.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.14.0
scipy: 1.0.0
pyarrow: 0.8.0
xarray: 0.10.0
IPython: 6.2.1
sphinx: 1.6.6
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: 1.5.1
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: 0.4.0
matplotlib: 2.1.2
openpyxl: 2.5.0
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.2
pymysql: 0.8.0
psycopg2: None
jinja2: 2.10
s3fs: 0.1.2
fastparquet: 0.1.4
pandas_gbq: None
pandas_datareader: None

</details>
