Fix randomly failing test in test_frame.py
@jreback I noticed a randomly failing test while doing a full suite run a week or two ago. The test is using random integers as part of a `MultiIndex`, which randomly leads to a non-unique index. Attempting to re-index the `DataFrame` therefore randomly fails. The solution is to just sample without replacement.

The following code gives a failure rate of ~1.5%, which is in line with birthday problem estimates (`d=1000` and four iterations each of `n=3` and `n=2`):

```
import subprocess

total = 0
n = 10
for i in range(n):
    result = subprocess.call('nosetests pandas/tests/test_frame.py:TestDataFrame.test_reindex_level', shell=True)
    if result:
        print i
        total += 1
print 'Total error rate of {0:.6f}'.format(total/float(n))
```
